jQuery's ajax method's success callback function takes three parameters. Here is the api .

success(data, textStatus, jqXHR);

So if you are making ajax call using jQuery the code might look like

$.ajax({
  url: "ajax/test.html",
  success: function (data, textStatus, jqXHR) {
    console.log(data);
  },
});

ajax using jquery-ujs

If you are using Rails and jquery-ujs then you might have code like this

<a href="/users/1" data-remote="true" data-type="json">Show</a>

$("a").bind("ajax:success", function (data, status, xhr) {
  alert(data.name);
});

Above code will not work. In order to make it work the very first element passed to the callback must be an event object. Here is the code that will work.

$("a").bind("ajax:success", function (event, data, status, xhr) {
  alert(data.name);
});

Remember that jQuery api says that the first parameter should be "data" then why we need to pass event object to make it work.

Why event object is needed

Here is snippet from jquery-ujs code

success: function(data, status, xhr) {
  element.trigger('ajax:success', [data, status, xhr]);
}

The thing about trigger method is that the event object is always passed as the first parameter to the event handler. This is why when you are using jquery-ujs you have to have the first parameter in the callback function an event object.

With jQuery it was easy.

After a few hours I had iconfirm.

This project is deprecated now.

return false

$("a").click(function () {
  console.log("clicked");
  return false;
});

First ensure that above code is executed on domready. Now if I click on any link then two things will happen.

e.preventDefault() will be called .
e.stopPropagation() will be called .

e.preventDefault()

As the name suggests, calling e.preventDefault() will make sure that the default behavior is not executed.

<a href='www.google.com'>click me</a>

If above link is clicked then the default behavior of the browser is to take you to www.google.com. However by invoking e.preventDefault() browser will not go ahead with default behavior and I will not be taken to www.google.com.

e.stopPropagation

When a link is clicked then an event "click event" is created. And this event bubbles all the way up to the top. By invoking e.stopPropagation I am asking browser to not to propagate the event. In other words the event will stop bubbling.

<div class="first">
  <div class="two">
    <a href="www.google.com">click me</a>
  </div>
</div>

If I click on "click me" then "click event" will start bubbling. Now let's say that I catch this event at .two and if I call e.stopPropagation() then this event will never reach to .first .

e.stopImmediatePropagation

First note that you can bind more than one event to an element. Take a look at following case.

<a class="one">one</a>

I am going to bind three events to the above element.

$("a").bind("click", function (e) {
  console.log("first");
});

$("a").bind("click", function (e) {
  console.log("second");
  e.stopImmediatePropagation();
});

$("a").bind("click", function (e) {
  console.log("third");
});

In this case there are three events bound to the same element. Notice that second event binding invokes e.stopImmediatePropagation() . Calling e.stopImmediatePropagation does two things.

Just like stopPropagation it will stop the bubbling of the event. So any parent of this element will not get this event.

However stopImmdiatePropagation stops the event bubbling even to the siblings. It kills the event right then and there. That's it. End of the event.

Once again calling stopPropagation means stop this event going to parent. And calling stopImmediatePropagation means stop passing this event to other event handlers bound to itself.

If you are interested here is link to DOM Level 3 Events spec.

Back to original problem

Now that I have described what preventDefault, stopPropagation and stopImmediatePropagation does lets see what changed in jQuery 1.4.3.

In jQuery 1.4.2 when I execute "return false" then that action was same as executing:

e.preventDefault();
e.stopPropagation();
e.stopImmediatePropagation();

Now e.stopImmediatePropagation internally calls e.stopPragation but I have added here for visual clarity.

Fact that return false was calling e.stopImmeidatePropagation was a bug. Get that. It was a bug which got fixed in jquery 1.4.3.

So in jquery 1.4.3 e.stopImmediatePropagation is not called. Checkout this piece of code from events.js of jquery code base.

if (ret !== undefined) {
  event.result = ret;
  if (ret === false) {
    event.preventDefault();
    event.stopPropagation();
  }
}

As you can see when return false is invoked then e.stopImmediatePropagation is not called.

It gets complicated with live and a bug in jQuery 1.4.3

To make the case complicated, jQuery 1.4.3 has a bug in which e.preventStopImmediatePropagation doest not work. Here is a link to this bug I reported.

To understand the bug take a look at following code:

<a href='' class='first'>click me</a>

$('a.first').live('click', function(e){
    alert('hello');
    e.preventDefault();
    e.stopImmediatePropagation();
});

$('a.first').live('click', function(){
    alert('world');
});

Since I am invoking e.stopImmediatePropagation I should never see alert world. However you will see that alert if you are using jQuery 1.4.3. You can play with it here .

This bug has been fixed as per this commit . Note that the commit mentioned was done after the release of jQuery 1.4.3. To get the fix you will have to wait for jQuery 1.4.4 release or use jQuery edge.

I am using rails.js (jquery-ujs). What do I do?

As I have shown "return false" does not work in jQuery 1.4.3 . However I would have to like to have as much backward compatibility in jquery-ujs as much possible so that the same code base works with jQuery 1.4 through 1.4.3 since not every one upgrades immediately.

This commit should make jquery-ujs jquery 1.4.3 compatible. Many issues have been logged at jquery-ujs and I will take a look at all of them one by one. Please do provide your feedback.

<div id="lab">
  <ul id="list">
    <li><a href="">USA</a></li>
  <ul>
  <p>
    <a href=''>USA</a>
  </p>
</div>

I want to get all links that contains the word USA. Simple enough. jQuery supports contains selector.

$(":contains('USA')");

Above query results in following items.

[html, body#body, div#lab, ul#list, li, a, ul, p, a]

That is because contains looks for given string under all the descendants.

has method to rescue

jQuery has method which returns the list of elements which have a descendant which has the given string.

b = $("*").has(":contains('USA')");

Above query results in following items.

[html, body#body, div#lab, ul#list, li, ul, p]

Final result

a = $(":contains('USA')");
b = $("*").has(":contains('USA')");
c = a.not(b);
console.log(c);

Above query results in following items.

 [a, a]

$("a:first").live("click", function () {
  log("clicked once");
  return false;
});
$("a:first").live("click", function () {
  log("clicked twice");
  return false;
});

I was expecting that I would see both the messages. However jQuery only invokes the very first message.

return false does two things. It stops the default behavior which is go and fetch the link mentioned in the href of the anchor tags. Also it stops the event from bubbling up. Since live method relies on event bubbling, it makes sense that second message does not appear.

Fix is simple. Just block the default action but let the event bubble up.

$("a:first").live("click", function (e) {
  log("clicked once");
  e.preventDefault();
});
$("a:first").live("click", function (e) {
  log("clicked twice");
  e.preventDefault();
});

$(function () {
  $(".fadein img:gt(0)").hide();
  setInterval(function () {
    $(".fadein :first-child")
      .fadeOut()
      .next("img")
      .fadeIn()
      .end()
      .appendTo(".fadein");
  }, 3000);
});

appendTo removes and attaches elements

In order to understand what's going on above, I am constructing a simple test page. Here is the html markup.

<div id="container">
  <div class="lab">This is div1</div>
  <div class="lab">This is div2</div>
</div>

Open this page in browser and execute following command in firebug.

$(".lab:first").appendTo("#container");

Run the above command 5/6 times to see its effect. Every single time you run JavaScript the order is changing.

The order of div elements with class lab is changing because if a jQuery element is already part of document and that element is being added somewhere else then jQuery will do cut and paste and not copy and paste . Again elements that already exist in the document get plucked out of document and then they are inserted somewhere else in the document.

Back to the original problem

In the original code the very first image is being plucked out of document and that image is being added to set again. In simpler terms this is what is happening. Initially the order is like this.

Image1
Image2
Image3

After the code is executed the order becomes this.

Image2
Image3
Image1

After the code is executed again then the order becomes this.

Image3
Image1
Image2

After the code is executed again then the order becomes this.

Image1
Image2
Image3

And this cycle continues forever.

However it is important to understand what goes on behind the scene for many reasons. And one of the important reasons is the performance of Rich Client. As more and more web pages use more and more jQuery code, understanding of how jQuery selects elements will speed up the loading of pages.

What is a selector engine

HTML documents are full of html markups. It's a tree like structure. Ideally speaking all the html documents should be 100% valid xml documents. However if you miss out on closing a div then browsers forgive you ( unless you have asked for strict parsing). Ultimately browser engine sees a well formed xml document. Then the browser engine renders that xml on the browser as a web page.

After a page is rendered then those xml elements are referred as DOM elements.

JavaScript is all about manipulating this tree structure (DOM elements) that browser has created in memory. A good example of manipulating the tree is command like the one give below which would hide the header element. However in order to hide the header tag, jQuery has to get to that DOM element.

jQuery("#header").hide();

The job of a selector engine is to get all the DOM elements matching the criteria provided by a user. There are many JavaScript selector engines in the market. Paul Irish has a nice article about JavaScript CSS Selector Engine timeline .

Sizzle is JavaScript selector engine developed by John Resig and is used internally in jQuery. In this article I will be showing how jQuery in conjunction with Sizzle finds elements.

Browsers help you to get to certain elements

Browsers do provide some helper functions to get to certain types of elements. For example if you want to get DOM element with id header then document.getElementById function can be used like this

document.getElementById("header");

Similarly if you want to collect all the p elements in a document then you could use following code .

document.getElementsByTagName("p");

However if you want something complex like the one given below then browsers were not much help. It was possible to walk up and down the tree however traversing the tree was tricky because of two reasons: a) DOM spec is not very intuitive b) Not all the browsers implemented DOM spec in same way.

jQuery("#header a");

Later selector API came out.

The latest version of all the major browsers support this specification including IE8. However IE7 and IE6 do not support it. This API provides querySelectorAll method which allows one to write complex selector query like document.querySelectorAll("#score>tbody>tr>td:nth-of-type(2)" .

It means that if you are using IE8 or current version of any other modern browser then jQuery code jQuery('#header a') will not even hit Sizzle. That query will be served by a call to querySelectorAll .

However if you are using IE6 or IE7, Sizzle will be invoked for jQuery('#header a'). This is one of the reasons why some apps perform much slower on IE6/7 compared to IE8 since a native browser function is much faster than elements retrieval by Sizzle.

Selection process

jQuery has a lot of optimization baked in to make things run faster. In this section I will go through some of the queries and will try to trace the route jQuery follows.

$('#header')

When jQuery sees that the input string is just one word and is looking for an id then jQuery invokes document.getElementById . Straight and simple. Sizzle is not invoked.

$('#header a') on a modern browser

If the browser supports querySelectorAll then querySelectorAll will satisfy this request. Sizzle is not invoked.

$('.header a[href!="hello"]') on a modern browser

In this case jQuery will try to use querySelectorAll but the result would be an exception (at least on firefox). The browser will throw an exception because the querySelectorAll method does not support certain selection criteria. In this case when browser throws an exception, jQuery will pass on the request to Sizzle. Sizzle not only supports css 3 selector but it goes above and beyond that.

$('.header a') on IE6/7

On IE6/7 querySelectorAll is not available so jQuery will pass on this request to Sizzle. Let's see a little bit in detail how Sizzle will go about handling this case.

Sizzle gets the selector string '.header a'. It splits the string into two parts and stores in variable called parts.

parts = [".header", "a"];

Next step is the one which sets Sizzle apart from other selector engines. Instead of first looking for elements with class header and then going down, Sizzle starts with the outer most selector string. As per this presentation from Paul Irish YUI3 and NWMatcher (Link is not available) also go right to left.

So in this case Sizzle starts looking for all a elements in the document. Sizzle invokes the method find. Inside the find method Sizzle attempts to find out what kind of pattern this string matches. In this case Sizzle is dealing with string a .

Here is snippet of code from Sizzle.find .

match: {
     ID: /#((?:[\w\u00c0-\uFFFF-]|\\.)+)/,
     CLASS: /\.((?:[\w\u00c0-\uFFFF-]|\\.)+)/,
     NAME: /\[name=['"]*((?:[\w\u00c0-\uFFFF-]|\\.)+)['"]*\]/,
     ATTR: /\[\s*((?:[\w\u00c0-\uFFFF-]|\\.)+)\s*(?:(\S?=)\s*(['"]*)(.*?)\3|)\s*\]/,
     TAG: /^((?:[\w\u00c0-\uFFFF\*-]|\\.)+)/,
     CHILD: /:(only|nth|last|first)-child(?:\((even|odd|[\dn+-]*)\))?/,
     POS: /:(nth|eq|gt|lt|first|last|even|odd)(?:\((\d*)\))?(?=[^-]|$)/,
     PSEUDO: /:((?:[\w\u00c0-\uFFFF-]|\\.)+)(?:\((['"]?)((?:\([^\)]+\)|[^\(\)]*)+)\2\))?/
},

One by one Sizzle will go through all the match definitions. In this case since a is a valid tag, a match will be found for TAG. Next following function will be called.

TAG: function(match, context){
     return context.getElementsByTagName(match[1]);
}

Now result consists of all a elements.

Next task is to find if each of these elements has a parent element matching .header. In order to test that a call will be made to method dirCheck. In short this is what the call looks like.

dir = 'parentNode';
cur = ".header"
checkSet = [ a www.neeraj.name, a www.google.com ] // object representation
dirCheck( dir, cur, doneName, checkSet, nodeCheck, isXML )

dirCheck method returns whether each element of checkSet passed the test. After that a call is made to method preFilter. In this method the key code is below

if ( not ^ (elem.className && (" " + elem.className + " ").replace(/[\t\n]/g, " ").indexOf(match) >= 0) )

For our example this is what is being checked

" header ".indexOf(" header ");

This operation is repeated for all the elements on the checkSet. Elements not matching the criteria are rejected.

More methods in Sizzle

if you dig more into Sizzle code you would see functions defined as +, > and ~ . Also you will see methods like

enabled: function(elem) {
          return elem.disabled === false && elem.type !== "hidden";
    },
disabled: function(elem) {
          return elem.disabled === true;
     },
checked: function(elem) {
          return elem.checked === true;
     },
selected: function(elem) {
          elem.parentNode.selectedIndex;
          return elem.selected === true;
     },
parent: function(elem) {
          return !!elem.firstChild;
     },
empty: function(elem) {
          return !elem.firstChild;
     },
has: function(elem, i, match) {
          return !!Sizzle( match[3], elem ).length;
     },
header: function(elem) {
          return /h\d/i.test( elem.nodeName );
     },
text: function(elem) {
          return "text" === elem.type;
     },
radio: function(elem) {
          return "radio" === elem.type;
     },
checkbox: function(elem) {
          return "checkbox" === elem.type;
     },
file: function(elem) {
          return "file" === elem.type;
     },
password: function(elem) {
          return "password" === elem.type;
     },
submit: function(elem) {
          return "submit" === elem.type;
     },
image: function(elem) {
          return "image" === elem.type;
     },
reset: function(elem) {
          return "reset" === elem.type;
     },
button: function(elem) {
          return "button" === elem.type || elem.nodeName.toLowerCase() === "button";
     },
input: function(elem) {
          return /input|select|textarea|button/i.test(elem.nodeName);
     }
},

first: function(elem, i) {
          return i === 0;
     },
last: function(elem, i, match, array) {
          return i === array.length - 1;
     },
even: function(elem, i) {
          return i % 2 === 0;
     },
odd: function(elem, i) {
          return i % 2 === 1;
     },
lt: function(elem, i, match) {
          return i < match[3] - 0;
     },
gt: function(elem, i, match) {
          return i > match[3] - 0;
     },
nth: function(elem, i, match) {
          return match[3] - 0 === i;
     },
eq: function(elem, i, match) {
          return match[3] - 0 === i;
     }

I use all these methods almost daily and it was good to see how these methods are actually implemented.

Performance Implications

Now that I have little more understanding of how Sizzle works, I can better optimize my selector queries. Here are two selectors doing the same thing.

$("p.about_me .employment");

$(".about_me  p.employment");

Since Sizzle goes from right to left, in the first case Sizzle will pick up all the elements with the class employment and then Sizzle will try to filter that list. In the second case Sizzle will pick up only the p elements with class employment and then it will filter the list. In the second case the right most selection criteria is more specific and it will bring better performance.

So the rule with Sizzle is to go more specific on right hand side and to go less specific on left hand side. Here is another example.

$(".container :disabled");

$(".container input:disabled");

The second query will perform better because the right side query is more specific.

$("#lab").animate({ height: "200px" }).hide();

If I pass a parameter to hide then it would start working.

$("#lab").animate({ height: "200px" }).hide(1);

As it turns out I did not have proper understanding of how effects work in jQuery.

animate method uses a queue inside. This is the queue to which all the pending activities are added.

$("#lab").animate({ height: "200px" }).animate({ width: "200px" });

In the above code element is being animated twice. However the second animation will not start until the first animation is done. While the first animation is happening the second animation is added to a queue. Name of this default queue is fx. This is the queue to which jQuery adds all the pending activities while one activity is in progress. You can inquire an element about how many pending activities are there in the queue.

$("#lab")
  .animate({ height: "200px" })
  .animate({ width: "200px" })
  .animate({ width: "800px" })
  .queue(function () {
    console.log($(this).queue("fx").length);
    $(this).dequeue();
  })
  .animate({ width: "800px" })
  .queue(function () {
    console.log($(this).queue("fx").length);
    $(this).dequeue();
  });

In the above code, twice the current queue is being asked to list number of pending activities. First time the number of pending activities is 3 and the second time it is 1.

Method show and hide also accepts duration. If a duration is passed then that operation is added to the queue. If duration is not passed or if the duration is zero then that operation is not added to queue.

$("#lab").hide(); // this action is not added to fx queue

$("#lab").hide(0); // this action is not added to fx queue

$("#lab").hide(1); // this action is added to fx queue

Coming back to the original question

When show or hide method is invoked without any duration then those actions are not added to queue.

$("#lab").animate({ height: "200px" }).hide();

In the above code since hide method is not added to queue, both the animate and the hide method are executed simultaneously. Hence the end result is that element is not hidden.

It could be fixed in a number of ways. One way would be to pass a duration to hide method.

$("#lab").animate({ height: "200px" }).hide(1);

Another way to fix it would be to pass hiding action as a callback function to animate method.

$("#lab").animate({ height: "200px" }, function () {
  $(this).hide();
});

Another way would be to explicitly put hide method in a queue.

$("#lab")
  .animate({ height: "200px" })
  .queue(function () {
    $(this).hide();
  });

Since hide method is not added to queue by default, in this case I have explicitly put the hide method to the queue.

Note that inside a queue method you must explicitly call dequeue for the next activity from the queue to be picked up.

$("#lab")
  .animate({ height: "200px" })
  .queue(function () {
    $(this).hide().dequeue();
  })
  .animate({ width: "200px" });

In the above code if dequeue is not called then second animation will never take place.

Also note that methods like fadeTo, fadeIn, fadeOut, slideDown, slideUp and animate are ,by default, added to default queue.

Turning off all animations

If for some reason you don't want animation then just set $.fx.off = true.

$.fx.off = true;
$("#lab").animate({ height: "200px" }, function () {
  $(this).hide();
});

Above code is telling jQuery to turn off all animations and that would result in the element hiding in an instant.

$('p').live('click', function({})

In the above case jQuery does nothing with the selected p elements. Since the result does not really matter, it is a good idea to remove such codes from the document ready callbacks list.

So instead of doing this

$(function(){
  $('p').live('click', function({})
})

just do this

$('p').live('click', function({})

Going a step further

John just landed this commit which adds delegate method .

Html markup

<div id="lab">
  <p>p inside lab</p>
</div>
<p>p outside lab</p>

If you want to track all the clicks on p then you could write like this.

$(document).delegate("p", "click", function () {
  log("p was clicked");
});

However if you only want to track clicks on 'p' which are inside the id lab then you can write like this.

$("#lab").delegate("p", "click", function () {
  log("p was clicked");
});

Note this functionality is in jQuery edge and is not available in jQuery 1.4.1. So you will have to get jQuery code from github to play with it.

If you are interested in the jQuery.live vs jQuery.fn.live discussion then follow this thread .

<style>
  p {
    display: inline;
  }
  #hello p {
    display: none;
  }
</style>

<div id="container">
  <div id="hello">
    Hello World
    <p>this is p inside hello</p>
  </div>
</div>

jQuery code.

$("p").show();

You can see the result here . Notice that when p is shown then display property of p is inline which is what it should be. All is well.

Now I'll change the css a little bit and will try the same code again. New css is .

<style>
  #container p {
    display: inline;
  }
  #hello p {
    display: none;
  }
</style>

See the result here . Notice that display property of p is block instead of inline .

Where did jQuery go wrong?

jQuery did not do anything wrong. It is just being a bit lazy. I'll explain.

Since the element was hidden when jQuery was asked to display it, jQuery had no idea where the element should have display property inline or block. So jQuery attempts to find out the display property of the element by asking browser what the display property should be.

jQuery first finds out the nodeName of the element. In this case value would be P. Then jQuery adds a P to body and then asks browser what is the display property of this newly added element. Whatever is the return value jQuery applies that value to the element that was asked to be shown.

In the first experiment, css style p { display: inline; }said that all p elements are inline. So when jQuery added a new p element to body and asked browser for the display property, browser replied 'inline' and 'inline' was applied to the element. All was good.

In the second case, I changed the stylesheet #container p { display: inline; } to have only p elements under id hello to have inline property. So when jQuery added a p element to body and asked for display type, browser correctly replied as 'block'.

So what's the fix.

Find the parent element (#hello) of the element in question ( p in this case) . jQuery should add a new p element to the #hello and then jQuery would get the right display property.

fadeTo method of jQuery ,strangely, fades event the hidden elements.

Here is html markup.

<style>
  #hello p {
    display: none;
  }
</style>
<div id="hello">
  <p>this is p inside hello</p>
</div>
<p>This is p outside hello</p>

Since the first p is hidden, you will see only one p element in the browser. Now execute following jQuery code.

$('p').fadeTo('slow', 0.5');

You will see both the p elements.

jQuery goes out of its way to make sure that hidden elements are visible. Here is fadeTo method.

fadeTo: function( speed, to, callback ) {
	return this.filter(":hidden").css("opacity", 0).show().end()
				.animate({opacity: to}, speed, callback);
}

Also notice that for a hidden element fadeTo operation starts with opacity of zero, while other elements will go down towards zero.

Checkout the same demo in slow motion and notice that while the first p element emerges out of hiding, the other p element is slowing fading. This might cause unwanted effect . So watch out for this one.

• First Demo
• Second Demo
• Third Demo

I will take a simple case of animating a div from height of 34 to 100.

Here is test case.

$(function () {
  $("#lab").css({ background: "yellow", height: "34px", margin: "10px" });

  $("a").click(function () {
    $("#lab").animate({ height: "100" });
    return false;
  });
});

Html markup is .

<a href="">click me</a>
<div id="lab">Hello World</div>

Inside the animate for each property an fx object is created.

jQuery.each( prop, function( name, val ) {
  var e = new jQuery.fx( self, opt, name );
}

Calling new on jQuery.fx returns a JavaScript object instance.

fx: function( elem, options, prop ) {
	this.options = options;
	this.elem = elem;
	this.prop = prop;

	if ( !options.orig ) {
		options.orig = {};
	}
}

Next in the animate method is a call to e.custom .

start = 34;
end = 100;
unit = "px";
e.custom(start, end, unit);

start, end and unit values are gleaned from the current state of div.

Here is custom method .

custom: function( from, to, unit ) {
	this.startTime = now();
	this.start = from;
	this.end = to;
	this.unit = unit || this.unit || "px";
	this.now = this.start;
	this.pos = this.state = 0;

	var self = this;
	function t( gotoEnd ) {
		return self.step(gotoEnd);
	}

	t.elem = this.elem;

	if ( t() && jQuery.timers.push(t) && !timerId ) {
		timerId = setInterval(jQuery.fx.tick, 13);
	}
},

As you can see every 13 milliseconds a call to step method is made.

step method is where real calculation is done. Here is the code.

step: function( gotoEnd ) {
  var t = now();
  var n = t - this.startTime;
  this.state = n / this.options.duration;
  pos = jQuery.easing['swing'](this.state, n, 0, 1, this.options.duration);
  this.now = this.start + ((this.end - this.start) * this.pos);
  this.update();
}

this.startTime is the time when the call to animate was invoked. The step method is called periodically from custom method. So the value of t is constantly changing. Based on the value of t, value of n will change. Some of the values of n I got was 1, 39, 69, 376 and 387.

While invoking animate method I did not specify a speed. jQuery picked up the default speed of 400. In this case the value of this.options.duration is 400. The value of state would change in each run and it would something along the line of 0.0025, 0.09, 0.265, 0.915 and 0.945 .

If you don't know what easing is then you should read this article by Brandon Aaron. Since I did not specify easing option, jQuery will pickup swing easing .

In order to get the value of next position this easing algorithm needs state, n and duration. When all of it was supplied then pos would be derived. The value of pos over the period of animation would change and it would be something like 0, 0.019853157161528467, 0.04927244144387716, 0.9730426794137726, 0.9973960708808632.

Based on the value of pos value of now is derived. And then update method is called to update the screen.

update method has following code that invokes _default method.

jQuery.fx.step._default)( this )

_default method has following code which finally updates the element.

fx.elem.style[fx.prop] = Math.max(0, fx.now);

fx.now value was set in the custom method and here that value was actually applied to the element.

You will have much better understanding of how animate works if you look at the source code. I just wanted to know at a high level what's going on and these are my findings.

jQuery makes it extremely easy to deal with JSON data. In the below example server sends a success message to browser. The JSON data looks like this.

{ 'success': 'record was successfully updated' }

The jQuery code to handle JSON data looks like this.

$.ajax({
  type: "GET",
  url: "test.js",
  dataType: "json",
  success: function (json) {
    $("#result").text(json.success);
  },
});

It all looks good and the code works with jQuery 1.3 .

However if you upgrade to jQuery 1.4 then above code will stop working. Why? jQuery 1.4 does strict JSON parsing using native parse method and any malformed JSON structure will be rejected.

How jQuery 1.3 parses JSON structure

jQuery 1.3 uses JavaScript’s eval to evaluate incoming JSON structure. Open firebug and type following example.

s = " { 'success' :  'record was updated' } ";
result = eval("(" + s + ")");
console.log(result);

You will get a valid output.

Note that all valid JSON structure is also valid JavaScript code so eval converts a valid JSON structure into a JavaScript object. However non JSON structure can also be converted into JavaScript object.

JSON specification says that all string values must use double quotes. Single quotes are not allowed. What it means is that following JSON structures are not valid JSON.

{ 'foo' : 'bar' }
{ foo: 'bar' }
{ foo: "bar" }
{ "foo" : 'bar' }

Even though above strings are not valid JSON if you eval them they will produce a valid JavaScript object. Since jQuery 1.3 uses eval on strings to convert JSON structure to JavaScript object all the above mentioned examples work.

However they will not work if you upgrade to jQuery 1.4 .

jQuery 1.4 uses native JSON parsing

Using eval to convert JSON into JavaScript object has a few issue. First is the security. It is possible that eval could execute some malicious code. Secondly it is not as fast as native parse methods made available by browsers. However browsers adhere to JSON spec and they will not parse malformed JSON structures. Open firebug and try following code to see how native browser methods do not parse malformed JSON structure. Here is the link to the announcement of Firefox support for native JSON parsing . John Resig mentioned the need for jQuery to have native JSON parsing support here .

s = " { 'success' :  'record was updated' } ";
result = eval("(" + s + ")");
console.log(result); /* returns valid JavaScript object */

result2 = window.JSON.parse(s);
console.log(result2); /* throws error */

As you can see a string which was successfully parsed by eval failed by window.JSON.parse . It might or might not fail in chrome. More on that later. Since jQuery 1.4 will rely on browsers parsing the JSON structure malformed JSON structures will fail.

In order to ensure that JSON is correctly parsed by the browsers, jQuery does some code cleanup to make sure that you are not trying to pass something malicious. You will not be able to test this thing directly using firebug but if you make an AJAX request and from server if you send response then you can verify the following code.

Following JSON structure will be correctly parsed in jQuery 1.3 . However the same JSON structure will fail in jQuery 1.4 . Why? Because of dangling open bracket [ .

' { "error" : "record was updated" }';

jQuery 1.4 has following code that does some data cleanup to get around the security issue with JSON parsing before sending that data to browser for parsing. Here is a snippet of the code.

// Make sure the incoming data is actual JSON
// Logic borrowed from http://json.org/json2.js
if (/^[\],:{}\s]*$/.test(data.replace(/\\(?:["\\\/bfnrt]|u[0-9a-fA-F]{4})/g, "@")
	.replace(/"[^"\\\n\r]*"|true|false|null|-?\d+(?:\.\d*)?(?:[eE][+\-]?\d+)?/g, "]")
	.replace(/(?:^|:|,)(?:\s*\[)+/g, "")))

Not all browsers parse JSON same way

Earlier I mentioned that following JSON structure will not be correctly parsed by browsers.

” { 'a':1 } ”

All browsers will fail to parse above JSON structure except chrome . Look at this blog titled Cross Browser JSON parsing to get more insight into this issue.

I have malformed JSON and I want to use jQuery 1.4

If you have malformed JSON and you want to use jQuery 1.4 then you should send the datatype as text and then convert the returned JSON structure using eval. Here is one way you can do that.

$.ajax({
  url: "/url",
  dataType: "text",
  success: function (data) {
    json = eval("(" + data + ")");
    // do something with json
  },
});

Ben Alman suggested another way in the comment section .

/* this should be the very first JavaScript inclusion file */
<script type=”text/javascript” language=”javascript”>window.JSON = null;</script>

jQuery attempts to parse JSON natively. However if native JSON parsing is not available then it falls back to eval. Here by setting window.JSON to null browser is faking that it does not have support for native JSON parsing.

Here are the two commits which made most of the changes in the way parsing is done.

Use JSONLint if you want to play with various strings to see which one is valid JSON and which one is not.

jQuery 1.4 fixed them all.

In this article I am going to discuss how jQuery brought support for these methods in jQuery. If you want a little background on what is live method and how it works then you should read this article which I wrote sometime back.

focus and blur events

IE and other browsers do not bubble focus and blur events. And that is in compliance with the w3c events model. As per the spec focus event and blur event do not bubble.

However the spec also mentions two additional events called DOMFocusIn and DOMFocusOut. As per the spec these two events should bubble. Firefox and other browsers implemented DOMFocusIn/DOMFocusOut . However IE implemented focusin and focusout and IE made sure that these two events do bubble up.

jQuery team decided to pick shorter name and introduced two new events: focusin and focusout. These two events bubble and hence they can be used with live method. This commit makes focusin/focusout work with live method. Here is code snippet.

if (document.addEventListener) {
  jQuery.each({ focus: "focusin", blur: "focusout" }, function (orig, fix) {
    jQuery.event.special[fix] = {
      setup: function () {
        this.addEventListener(orig, handler, true);
      },
      teardown: function () {
        this.removeEventListener(orig, handler, true);
      },
    };

    function handler(e) {
      e = jQuery.event.fix(e);
      e.type = fix;
      return jQuery.event.handle.call(this, e);
    }
  });
}

Once again make sure that you are using focusin/focusout instead of focus/blur when used with live .

mouseenter and mouseleave events

mouseenter and mouseleave events do not bubble in IE. However mouseover and mouseout do bubble in IE. If you are not sure of what the difference is between mouseenter and mouseover then watch this excellent screencast by Ben.

The fix that was applied to map for focusin can be replicated here to fix mousetner and mouseleave issue. This is the commit that fixed mouseenter and mouseleave issue with live method.

jQuery.each(
  {
    mouseenter: "mouseover",
    mouseleave: "mouseout",
  },
  function (orig, fix) {
    jQuery.event.special[orig] = {
      setup: function (data) {
        jQuery.event.add(
          this,
          fix,
          data && data.selector ? delegate : withinElement,
          orig
        );
      },
      teardown: function (data) {
        jQuery.event.remove(
          this,
          fix,
          data && data.selector ? delegate : withinElement
        );
      },
    };
  }
);

Event detection

Two more events are left to be handled: submit and change. Before jQuery applies fix for these two events, jQuery needs a way to detect if a browser allows submit and change events to bubble or not. jQuery team does not favor browser sniffing. So how to go about detecting event support without browser sniffing.

Juriy Zaytsev posted an excellent blog titled Detecting event support without browser sniffing . Here is the short and concise way he proposes to find out if an event is supported by a browser.

var isEventSupported = (function () {
  var TAGNAMES = {
    select: "input",
    change: "input",
    submit: "form",
    reset: "form",
    error: "img",
    load: "img",
    abort: "img",
  };
  function isEventSupported(eventName) {
    var el = document.createElement(TAGNAMES[eventName] || "div");
    eventName = "on" + eventName;
    var isSupported = eventName in el;
    if (!isSupported) {
      el.setAttribute(eventName, "return;");
      isSupported = typeof el[eventName] == "function";
    }
    el = null;
    return isSupported;
  }
  return isEventSupported;
})();

In the comments section John Resig mentioned that this technique can also be used to find out if an event bubbles or not.

John committed following code to jQuery.

var eventSupported = function (eventName) {
  var el = document.createElement("div");
  eventName = "on" + eventName;

  var isSupported = eventName in el;
  if (!isSupported) {
    el.setAttribute(eventName, "return;");
    isSupported = typeof el[eventName] === "function";
  }
  el = null;

  return isSupported;
};

jQuery.support.submitBubbles = eventSupported("submit");
jQuery.support.changeBubbles = eventSupported("change");

Next task is to actually make a change event or a submit event bubble if ,based on above code, it is determined that browse is not bubbling those events .

Making change event bubble

On a form a person can change so many things including checkbox, radio button, select menu, textarea etc. jQuery team implemented a full blown change tracker which would detect every single change on the form and will act accordingly.

Radio button, checkbox and select changes will be detected via change event. Here is the code.

click: function( e ) {
	var elem = e.target, type = elem.type;

	if ( type === "radio" || type === "checkbox" || elem.nodeName.toLowerCase() === "select" ) {
		return testChange.call( this, e );
	}
},

In order to detect changes on other fields like input field, textarea etc keydown event would be used. Here it the code.

keydown: function( e ) {
	var elem = e.target, type = elem.type;

	if ( (e.keyCode === 13 && elem.nodeName.toLowerCase() !== "textarea") ||
		(e.keyCode === 32 && (type === "checkbox" || type === "radio")) ||
		type === "select-multiple" ) {
		return testChange.call( this, e );
	}
},

IE has a proprietary event called beforeactivate which gets fired before any change happens. This event is used to store the existing value of the field. After the click or keydown event the changed value is captured. Then these two values are matched to see if really a change has happened. Here is code for detecting the match.

function testChange(e) {
  var elem = e.target,
    data,
    val;

  if (!formElems.test(elem.nodeName) || elem.readOnly) {
    return;
  }

  data = jQuery.data(elem, "_change_data");
  val = getVal(elem);

  if (val === data) {
    return;
  }

  // the current data will be also retrieved by beforeactivate
  if (e.type !== "focusout" || elem.type !== "radio") {
    jQuery.data(elem, "_change_data", val);
  }

  if (elem.type !== "select" && (data != null || val)) {
    e.type = "change";
    return jQuery.event.trigger(e, arguments[1], this);
  }
}

Here is the commit that fixed this issue.

jQuery.event.special.change = {
	filters: {
		focusout: testChange,

		click: function( e ) {
			var elem = e.target, type = elem.type;

			if ( type === "radio" || type === "checkbox" || elem.nodeName.toLowerCase() === "select" ) {
				return testChange.call( this, e );
			}
		},

		// Change has to be called before submit
		// Keydown will be called before keypress, which is used in submit-event delegation
		keydown: function( e ) {
			var elem = e.target, type = elem.type;

			if ( (e.keyCode === 13 && elem.nodeName.toLowerCase() !== "textarea") ||
				(e.keyCode === 32 && (type === "checkbox" || type === "radio")) ||
				type === "select-multiple" ) {
				return testChange.call( this, e );
			}
		},

		// Beforeactivate happens also before the previous element is blurred
		// with this event you can't trigger a change event, but you can store
		// information/focus[in] is not needed anymore
		beforeactivate: function( e ) {
			var elem = e.target;

			if ( elem.nodeName.toLowerCase() === "input" && elem.type === "radio" ) {
				jQuery.data( elem, "_change_data", getVal(elem) );
			}
		}
	},
	setup: function( data, namespaces, fn ) {
		for ( var type in changeFilters ) {
			jQuery.event.add( this, type + ".specialChange." + fn.guid, changeFilters[type] );
		}

		return formElems.test( this.nodeName );
	},
	remove: function( namespaces, fn ) {
		for ( var type in changeFilters ) {
			jQuery.event.remove( this, type + ".specialChange" + (fn ? "."+fn.guid : ""), changeFilters[type] );
		}

		return formElems.test( this.nodeName );
	}
};

var changeFilters = jQuery.event.special.change.filters;

}

Making submit event bubble

In order to detect submission of a form, one needs to watch for click event on a submit button or an image button. Additionally one can hit 'enter' using keyboard and can submit the form. All of these need be tracked.

jQuery.event.special.submit = {
  setup: function (data, namespaces, fn) {
    if (this.nodeName.toLowerCase() !== "form") {
      jQuery.event.add(this, "click.specialSubmit." + fn.guid, function (e) {
        var elem = e.target,
          type = elem.type;

        if (
          (type === "submit" || type === "image") &&
          jQuery(elem).closest("form").length
        ) {
          return trigger("submit", this, arguments);
        }
      });

      jQuery.event.add(this, "keypress.specialSubmit." + fn.guid, function (e) {
        var elem = e.target,
          type = elem.type;

        if (
          (type === "text" || type === "password") &&
          jQuery(elem).closest("form").length &&
          e.keyCode === 13
        ) {
          return trigger("submit", this, arguments);
        }
      });
    }
  },

  remove: function (namespaces, fn) {
    jQuery.event.remove(
      this,
      "click.specialSubmit" + (fn ? "." + fn.guid : "")
    );
    jQuery.event.remove(
      this,
      "keypress.specialSubmit" + (fn ? "." + fn.guid : "")
    );
  },
};

As you can see if a submit button or an image is clicked inside a form the submit event is triggered. Additionally keypress event is monitored and if the keyCode is 13 then the form is submitted.

live method is just pure awesome. It is great to see last few wrinkles getting sorted out. A big Thank You to Justin Meyer of JavaScriptMVC who submitted most of the patch for fixing this vexing issue.

$(".coda-slider-wrapper ul a.current").parent().next().find("a").click();

with this code

var direction = "next";
$(".coda-slider-wrapper ul a.current").parent()[direction]().find("a").click();

I had never seen anything like that. In the above mentioned article, Remi used next and prev methods. However I wanted to know all the options I could pass since this feature is not very documented.

Snippet from jQuery source code

Here is code from jQuery that makes that above method work.

jQuery.each(
  {
    parent: function (elem) {
      return elem.parentNode;
    },
    parents: function (elem) {
      return jQuery.dir(elem, "parentNode");
    },
    next: function (elem) {
      return jQuery.nth(elem, 2, "nextSibling");
    },
    prev: function (elem) {
      return jQuery.nth(elem, 2, "previousSibling");
    },
    nextAll: function (elem) {
      return jQuery.dir(elem, "nextSibling");
    },
    prevAll: function (elem) {
      return jQuery.dir(elem, "previousSibling");
    },
    siblings: function (elem) {
      return jQuery.sibling(elem.parentNode.firstChild, elem);
    },
    children: function (elem) {
      return jQuery.sibling(elem.firstChild);
    },
    contents: function (elem) {
      return jQuery.nodeName(elem, "iframe")
        ? elem.contentDocument || elem.contentWindow.document
        : jQuery.makeArray(elem.childNodes);
    },
  },
  function (name, fn) {
    jQuery.fn[name] = function (selector) {
      var ret = jQuery.map(this, fn);

      if (selector && typeof selector == "string")
        ret = jQuery.multiFilter(selector, ret);

      return this.pushStack(jQuery.unique(ret), name, selector);
    };
  }
);

As you can see, a variety of selectors can be passed to jQueryCollection[].

If you want to give a try, any jQuery enabled site should perform all of the below mentioned code without any problem.

var a = $("a:first");
var log = console.log;

log(a["parent"]());
log(a["parents"]());
log(a["next"]());
log(a["prev"]());
log(a["nextAll"]());
log(a["prevAll"]());
log(a["siblings"]());
log(a["children"]());
log(a["contents"]());

<div>
  <p>This is p</p>
</div>

Following jQuery code will do the work.

$(document).ready(function () {
  var div = $("<div></div>");
  var p = $("<p></p>").text("this is p").appendTo(div);

  $("body").append(div);
});

A better way to accomplish the same is presented below.

$("<div></div>")
  .append("<p></p>")
  .find("p")
  .text("this is p")
  .end()
  .appendTo("body");

Using .end() you can go back one level. And you can use .end() any number of times to get out of a deeply nested tag.

$("<div></div>")
  .append("<p></p>")
  .find("p")
  .append("<span></span>")
  .find("span")
  .text("this is span")
  .end()
  .end()
  .appendTo("body");

All the JavaScript code mentioned in this blog should be tried on firebug.

The super popular live method was added to jQuery 1.3 . It works just great. Except when it does not work. As per the documentation this method does not work in following cases: blur, focus, mouseenter, mouseleave, change, submit .

How binding events work in jQuery

$("a").click(function () {
  console.log("clicked");
});

In above case click event is bound to all the links in the document. jQuery stores such binding information as data attribute of the bound element. This is how I can access the list of all functions bound to an element.

$("a").data("events"); // Object click=Object

If a new link is dynamically added then that new link will not get this click behavior. To solve this problem I can use live method.

Trying out live event

$("a").live("click", function () {
  console.log("clicked");
});

If I add a new a tag dynamically then that tag will automatically get the new click behavior. That's great.

Just like the previous section, now I am going to find the events bound to a element. However when I execute following code I get undefined .

$('a').data('events')); //undefined

Why is that. In the previous section I showed that all the events bound to an element are stored as the data attribute of that element. Well, live is different. live events are not bound to the element directly. They are bound to the top level 'document'. I can verify this by trying out this code

jQuery.data(document, "events").live; //Object

How does live method work

live methods do not set anything on elements directly. All the event handlers are set at the document level. It means that in order for live methods to work, event bubbling is required. If you don't know what event bubbling is then read her . It also means that event should not be stopped while it is propagating to document. If event propagation is stopped then event handlers bound at the document level will never know about that event and live method will fail.

The strategy to let someone else deal with the event is called event delegation.

When live method is called then a binding is done at the document level. Loosely translated this is what is basically happening.

$(document).bind("click", function (event) {
  var $target = $(event.target);
  if ($target.is("p")) {
    console.log("p was clicked");
  }
});

As you can see when a click on p event bubbles all the way to the top then that event is captured by document and necessary action is taken if the target element matches.

It is clear that if the click event is stopped before it reaches document then live method will not work. I will show you an example.

<div id="parent">
  Languages
  <p>Java</p>
  <p>Javascript</p>
</div>

$("p").live("click", function (e) {
  console.log("p was clicked");
});

If I click on Java or Javascript I get a message on console. live is working great. Now I'll stop the event propagation when event reaches to div.

$("p").live("click", function (e) {
  console.log("p was clicked");
});

$("#parent").click(function (e) {
  console.log("stopping propagation");
  e.stopPropagation();
});

Now you will notice that live is no more working.

live does not work when event bubbling is not supported

In the previous section I showed that when event does not bubble up to document then live fails. It means that all the events that do not bubble will not work. Which means that events like blur, focus, mouseenter, mouseleave, change and submit which do bubble in IE will not work in IE. However note that these events will continue to work in Firefox.

<select id="lab1a" name="sweets" multiple="multiple">
  <option>Chocolate</option>
  <option selected="selected">Candy</option>
  <option>Taffy</option>
  <option selected="selected">Caramel</option>
  <option>Fudge</option>
  <option>Cookie</option>
</select>
<div id="lab1b" style="color:red;"></div>

$("#lab1a").live("change", function () {
  var str = "";
  $("select option:selected").each(function () {
    str += $(this).text() + " ";
  });
  $("#lab1b").text(str);
});

Above code will work in firefox but it will not work in IE.

Here is an example of an event that does not work in IE: onchange event.

DOM models suggest that onchange event should bubble . However msdn documentation clearly says that onchange event does not bubble .

Make all the live method problems go away

To recap live method will not work in following cases:

• live method works on event propagation, if an event is stopped while it is bubbling then live will not work.
• IE does not support bubbling for certain events. live method on those events will not work in IE.

There is a way to get around to both the problems.

Brandon Aaron developed livequery plugin which was finally merged into jQuery as live method. livequery plugin solves both the problems listed above and the code works in IE too.

First step is to include this plugin

<script
  src="http://github.com/brandonaaron/livequery/raw/master/jquery.livequery.js"
  type="text/javascript"
></script>

Now try this code.

$("p").livequery("click", function (e) {
  alert("p was clicked");
});

$("#parent").click(function (e) {
  alert("stopping progpagation");
  e.stopPropagation();
});

$("#lab1a").livequery("change", function () {
  var str = "";
  $("select option:selected").each(function () {
    str += $(this).text() + " ";
  });
  $("#lab1b").text(str);
});

<select id="lab1a" name="sweets" multiple="multiple">
  <option>Chocolate</option>
  <option selected="selected">Candy</option>
  <option>Taffy</option>
  <option selected="selected">Caramel</option>
  <option>Fudge</option>
  <option>Cookie</option>
</select>
<div id="lab1b" style="color:red;"></div>
<div id="parent">
  Languages
  <p>Java</p>
  <p>Javascript</p>
</div>

If I click on 'Java' or 'Javascript' I will get proper response even though event propagation is being stopped. Also even though change event does not bubble in IE, above code works in IE.

livequery works because livequery ,unlike live, method does not do binding at the document level. livequery does binding at the element level. You can find that out by running following code.

$("p").data("events");

Above code will produce result if I am using livequery. Above code will not produce any result if I am using live method.

The key piece of code in the plugin that makes all this work is

// Create a timeout to check the queue and actually run the Live Queries
$.livequery.timeout = setTimeout($.livequery.checkQueue, 20);

Every 20 milliseconds livequery runs all the bindings defined in livequery and then binds the matched element with the event.

By understanding the internals of how live and livequery are implemented, now you can choose to use livequery in certain cases where live will not work. Also it helps to understand how live actually works.

Live method finds elements and then throws it away. Not very efficient.

A typical use of live method is something like this.

$("p").live("click", function (e) {
  console.log("p was clicked");
});

As it has already been discussed a live method registers events at document level.

However when $('p').live(...) is evaluated then jQuery first goes and finds all the p elements. And then what does it do with those elements. Nothing. That's right. jQuery throws away all those p elements which were just found without using them. What a waste.

If your application has a lot of live methods and this might slow down the performance of the application.

A better solution would have been to design an API like this one:

$.live('p', 'click', function(){..});~~~

jQuery is flexible and I can create my own live method but it will add to the confusion. Another solution would be to make the call to live method without first finding all those `p` element. Here is how it can be done.

~~~javascript
var myDocument = $(document);
myDocument.selector = 'p';
myDocument.live('click', function(){
  console.log('p was clicked');
});

In the above case no element will be selected only to be thrown away. This is much better.

Seeing is believing

In the previous section, I showed how live method can be made to work without first selecting the elements. However a friend of mine asked me if I could conclusively prove that in the real live method a find is actually done. And in my solution a find call is not done.

Here I am showing you the overriding find method. In this method I am overriding the original find method. I will put a log message before passing the find method to the original method.

(function () {
  var originalFindMethod = jQuery.fn.find;
  jQuery.fn.find = function () {
    console.log("find was called");
    originalFindMethod.apply(this, arguments);
  };
})();

$(document).ready(function () {
  $("p").live("click", function () {
    console.log("p was clicked");
  });
});

In the above case you will get message on firebug console confirming that find is indeed invoked when live method is called.

Here is revised version of live. Try this one.

(function () {
  var originalFindMethod = jQuery.fn.find;
  jQuery.fn.find = function () {
    console.log("find was called");
    originalFindMethod.apply(this, arguments);
  };
})();

$(document).ready(function () {
  var myDocument = $(document);
  myDocument.selector = "p";
  myDocument.live("click", function () {
    console.log("p was clicked");
  });
});

Above version does not print 'find was called'.

I have following code which depends on jQuery.

var Search = {
  initAction: function ($elm) {
    if ($elm.attr("value").length === 0) {
      $elm.attr("value", "search");
    }
  },

  blurAction: function ($elm) {
    if ($elm.attr("value") === "search") {
      $elm.attr("value", "");
    }
  },
};

Search.initAction($("#query_term_input"));
Search.blurAction($("#query_term_input"));

Everything is cool.

Next, company decides to use a cool JavaScript widget that depends on Prototype library. After adding the Prototype library my code starts failing. I have been asked to fix my code.

I can obviously go through the code and do a mass find $ and replace $ with jQuery. This is error prone.

A better solution would be to make use of self invoking function and redefine $ to jQuery .

var Search = (function ($) {
  return {
    initAction: function ($elm) {
      if ($elm.attr("value").length === 0) {
        $elm.attr("value", "search");
      }
    },

    blurAction: function ($elm) {
      if ($elm.attr("value") === "search") {
        $elm.attr("value", "");
      }
    },
  }; // return
})(jQuery);

Search.initAction(jQuery("#query_term_input"));
Search.blurAction(jQuery("#query_term_input"));

var foo = {
  first: function () {
    console.log("I am first");
  },

  second: function () {
    console.log("I am second");
  },
};

foo.first();
foo.second();

Everything works fine.

Now there is a need for function second to call function first. Try this and it will fail.

var foo = {
  first: function () {
    console.log("I am first");
  },

  second: function () {
    console.log("I am second");
    first();
  },
};

foo.second();

One way to fix this problem is to hardcode value foo inside the second function. Following code works.

var foo = {
  first: function () {
    console.log("I am first");
  },

  second: function () {
    console.log("I am second");
    foo.first();
  },
};

foo.second();

Above code works but hardcoding the value of foo inside the object literal is not good. A better way would be to replace the hardcoded name with this .

var foo = {
  first: function () {
    console.log("I am first");
  },

  second: function () {
    console.log("I am second");
    this.first();
  },
};

foo.second();

All is good.

Chasing this

Javascript allows one to create function inside a function. Now I am changing the implementation of method second. Now this method would return another function.

var foo = {
  first: function () {
    console.log("I am first");
  },

  second: function () {
    return function () {
      this.first();
    };
  },
};

foo.second()();

Also note that in order to invoke the returned function I have double ()() at the end.

Above code does not work. It did not work because this has changed. Now this in second function refers to the global object rather than referring to the foo object.

Fix is simple. In the function second store the value of this in a temporary variable and the inner function should use that temporary variable. This solution will work because of Javascript's inherent support for closure.

var foo = {
  first: function () {
    console.log("I am first");
  },

  second: function () {
    var self = this;
    return function () {
      self.first();
    };
  },
};

foo.second()();

Javascript is all about interactions. When an event happens then something should happen. For example hover, click, focus, mouseover etc are basic events which are used on a regular basis. jQuery provides a method called bind to bind an action to an event. For example, if we want an alert message when we click on a link then that can be done like this.

$("a").bind("click", function () {
  alert("I have been clicked");
});

jQuery also allows developers to make use of custom events. How it is going to help us, we are going to see it shortly. First let's take a look at a basic calendar application.

This is a simple javascript calendar which primarily does four things: render calendar itself, pull data from API and update calendar with API data, ability to collapse event information and ability to expand event information.

Code looks like this.

function redrawForMonth(d) {
  //business logic to render calendar
  $("#monthly_calendar_umbrella").updateCalendarWithAPIData();
  $("#monthly_calendar_umbrella").ExpandAllEvents();
}

function updateCalendarWithAPIData() {
  //business logic to get the data from API and appending data to appropriate date cell
}

function CollapseAllEvents() {
  //business logic
}

function ExpandAllEvents() {
  //business logic
}

In the above case we have four methods. If all the implementation is filled out and helper methods are added then ,in total, we'll have tons of methods.

And slowly, it'll become difficult to see which methods act on the main element monthly_calendar_umbrella.

Let's look at the same functionality if implemented using jQuery custom events.

$("#monthly_calendar_umbrella")
  .bind("redrawForMonth", function (e, d) {})

  .bind("updateCalendarWithAPIData", function (e, d) {})

  .bind("CollapseAllEvents", function (e) {})

  .bind("ExpandAllEvents", function (e) {});

First difference, you will notice is how nicely all the actions possible on the main element are laid out. In this case just one look at the code tells me that four actions can be performed on the main element. This was not so obvious from the first version of the code.

In this case, I am binding events such as 'redrawFroMonth' to the element. Obviously 'redrawForMonth' is not a native event such as 'click' or 'submit'. This is what I mean by binding 'custom events' to elements. In this case 'redrawForMonth' is a custom event.

The other thing that is not so obvious is the shift in focus. Traditional javascript programming has been too obsessed with the elements that is clicked or submitted that causes an action. The emphasis has been too much on keeping the code around the element that creates an action. In this case the code has been developed around the element that is being acted upon.

Now, the last part of the discussion is how to trigger custom events. Well, jQuery has a method called trigger .

Note that while binding a function is passed. The first parameter of that function is always an event handler. In the below example I am passing only one parameter even though the function takes two parameters: event handler element and the parameter I defined.

$("#monthly_calendar_umbrella").trigger("redrawForMonth", new Date());

Recently I was discussing how to create a jQuery plugin with a friend of mine. In the end I had a base code that should work as a starting point of any jQuery plugin.

Take a look here .

Let's say that I have bound all the links to display an alert.

$("a").bind("click", function (e) {
  e.preventDefault();
  alert("clicked");
});

Mine is a large application and a co-worker has added another javascript file which does this

$("a").bind("click", function (e) {
  e.preventDefault();
  alert("hello");
});

Now if I click on a link I get two alerts. Not good. One way to debug would be to go through all the included javascript files.

However it would be cool if there is a way to find all the click handlers associated with an element.

jQuery has data method which it uses internally to store information. jQuery uses data method to store all the handlers associated with an element. We could use this information to our advantage. Here I am trying to find out all the click handlers associated with the first link.

var output = jQuery.data($("a").get(0), "events");

jQuery.each(output.click, function (key, value) {
  alert(value);
});

The output looks like this

function (e) { e.preventDefault(); alert("clicked"); }
function (e) { e.preventDefault(); alert("hello"); }

jQuery.data method is also very useful if you want to store some information about an element. For example, in an application you need to store information about people. In the html page you are only displaying names. And when a name is clicked then you want to display age, hometown and the company they work for. You can store information about each user using jQuery.data and you can retrieve it when username is clicked.

<div id="parent">
  Languages
  <p>Java</p>
  <p>Javascript</p>
</div>

And here is the javascript code. Run the code with jQuery-1.3.2 .

$(document).ready(function () {
  $("#parent").click(function () {
    alert("parent was clicked");
  });
  $("#parent p").click(function () {
    alert("p was clicked");
  });
});

When you click on Java then you will get two alerts. That is because the click on element p will propagate outward and it will be caught by element div which has onclick trigger.

If you do not want the event to propagate then here is a way to stop it.

$(document).ready(function () {
  $("#parent").click(function () {
    alert("parent was clicked");
  });
  $("#parent p").click(function (e) {
    alert("p was clicked");
    e.stopPropagation();
  });
});

Stopping the default behavior

Converting a regular html link into an AJAX request is easy. One of the things you need to do is to return false so that the click operation does not perform its default behavior.

$("a").bind("click", function () {
  alert("clicked");
  return false;
});

However there is another way to handle such cases. preventDefault stops the default behavior. The same code could be written as

$("a").bind("click", function (e) {
  e.preventDefault();
  alert("clicked");
});

Not sure why but I have noticed that e.preventDefault() should be the first line in the function. If I switch the order of e.preventDefault and alert message in the above javascript then it does not work in Firefox/mac.

function foo() {
  var x = 100;
  alert(x);

  for (var x = 1; x <= 10; x++) {}

  alert(x);
}

foo();

What do you think the value of second alert will be. I thought it would be 100 but the answer is 11. That's because the scope of a local variable in Javascript is not limited to the loop. Rather the scope of a local variable is felt all over the function.

Before you look at the next piece of code remember that defining a variable without the var prefix makes that variable a global variable.

fav_star = "Angelina"; /* it is a global variable */

function foo() {
  var message = "fav star is " + fav_star;
  alert(message);

  var fav_star = "Jennifer";
  var message2 = "fav star is " + fav_star;
  alert(message2);
}

foo();

What do you think would be the alert value first time? I thought it would be Angelina but the correct answer is undefined. That is because it does not matter where the variable is defined. As long as the variable is defined anywhere within a function then that variable will be set to undefined. It is when the statement is executed then the value of the variable changes from undefined to Jennifer.

Thanks to my friend Subba Rao for bringing this feature of Javascript to my attention and for discussing it.

What are we talking about

My main concern is that as Javascript developer I should not be polluting global namespace. I could write the calendar code like this.

showCalendar = function (options, d) {};

calendarHeaderMonth = function (opts) {};

calendarHeaderWeek = function (opts) {};

calendarData = function (opts) {};

drawCalendar = function (opts, d) {};

getCellID = function (year, month, day) {};

// use showCalendar
showCalendar({}, new Date());

You can see that the main method that I want to expose to global namespace is showCalendar. However I am also exposing six other functions to global namespace. It means that if some user has declared a global namespace function named getCellID then my getCellID is going to collide with the user's function. This is not good.

Goal

The goal is to refactor the code in such a way that only one method showCalendar is exposed to the global namespace. In order to test the code at the very end of the javascript I will invoke a call to method getCellID.

showCalendar = function (options, d) {};

calendarHeaderMonth = function (opts) {};

calendarHeaderWeek = function (opts) {};

calendarData = function (opts) {};

drawCalendar = function (opts, d) {};

getCellID = function (year, month, day) {};

// use showCalendar
showCalendar({}, new Date());

// this call should fail but is not failing now
getCellID(2009, 5, 4);

The above call to getCellID is currently being executed successfully. It means I have polluted the global namespace. I am going to fix it.

Solution

The solution is to put all the code inside a function. In a function all the variables and functions declared are scoped only to that function. Out of that function, inner functions cannot be used. Inside this function only one method showCalendar would be exposed.

Here is the solution.

(function () {
  // by not declaring var this variable is going to be at global namespace
  showCalendar = function (options, d) {};

  var calendarHeaderMonth = function (opts) {};

  var calendarHeaderWeek = function (opts) {};

  var calendarData = function (opts) {};

  var drawCalendar = function (opts, d) {};

  var getCellID = function (year, month, day) {};
})();

showCalendar({}, new Date());

// this call will fail
getCellID(2009, 5, 4);

In the above case the call to showCalendar was successful. However call to getCallID failed. That's good. It means I am not polluting the global namespace.

If you notice carefully in the above case after declaring an anonymous function I am invoking that function by having () at the end. You can read more about self-invoking function here .

Here is a great article which describes in detail how to wrap your javascript code inside a jQuery plugin. The article did a great job of explaining the basics however it took me a while to understand the self-invoking functions.

Self-invoking functions

self-invoking functions are anonymous functions declared on run time and then invoke it right then and there. Since they are anonymous functions they can't be invoked twice. However they are a good candidate for initialization work which is exactly what is happening in the jQuery plugin pattern.

Declaring a function

A function can be declared in two ways:

function hello() {
  alert("Hello");
}
var hello = function () {
  alert("Hello");
};

The end result in the above two javascript statement was same. In the end a variable named hello was created which is a function.

Please note that in the above case only the functions are declared. They are not invoked. In order to invoke the function we have to do this

var hello = function () {
  alert("Hello");
};
hello();

How do we invoke an anonymous function. We need to call () on the anonymous function. But before that we need to wrap the whole command in (). Take a look at this

// original
function hello() {
  alert("Hello");
}

// step1 wrap everything in () so that we have a context to invoke
(function hello() {
  alert("Hello");
})(
  // now call () to invoke this function
  function hello() {
    alert("Hello");
  }
)();

The final result is weird looking code but it works and that is how an anonymous function is declared and invoke on run time.

With this understanding now it becomes easier to see what is happening in this code

(function ($) {
  // ....
})(jQuery);

In the above case an anonymous function is being created. However unlike my anonymous function this anonymous function take a parameter and the name of this parameter is $. $ is nothing else but jQuery object that is being passed while invoking this function.