How does jQuery accomplish chaining of commands?

Question

I consider myself (at best) a mid-level JavaScript guy...and of course...I want to understand HOW some things are accomplished so I can write better code. As such, I've been looking under-the-hood of jQuery trying to uderstand it a little more about how certain things are accomplished.

For example:
jQuery handles the case where IE and Opera return items by name instead of ID by doing the following:

// HANDLE: $("#id")
else 
{
    var elem = document.getElementById( match[3] );

    // Handle the case where IE and Opera return items
    // by name instead of ID
    if ( elem && elem.id != match[3] )
        return jQuery().find( selector );

    // Otherwise, we inject the element directly into the jQuery object
    var ret = jQuery( elem || [] )
    ret.context = document;
    ret.selector = selector;

    return ret;
}

...okay that's easy!

But a typical line of jQuery code chains a series of commands together:

$('#someElementId').fadeIn().css({ ...Some CSS Properties ... });

Now...I 'think' the chaining portion is handled in the following area:

jQuery.extend = jQuery.fn.extend = function() { ... };

I'm combing through this area...and I see HOW a single command is processed...but I don't really see WHERE or HOW a chain-of-commands are managed. So I am obviously missing something.

So My Question Is:

How does jQuery implement chaining of selectors?

Each method returns the current or modified selection object. It's called Fluent Interface style. http://en.wikipedia.org/wiki/Fluent_interface#JavaScript — asawyer, Dec 08 '11 at 13:24
Okay...that's cool. But can you please show AND explain where this is going on in the jQuery code? — Prisoner ZERO, Dec 08 '11 at 13:26
The link has examples in multiple languages, including javascript. — asawyer, Dec 08 '11 at 13:27
Okay...I see. If you identify WHERE this happens in the code and put it into an answer below (and maybe add some commenting) then I can credit you. — Prisoner ZERO, Dec 08 '11 at 13:33

score 3 · Answer 1 · answered Dec 08 '11 at 13:27

3

All the methods on a jQuery object will return a jQuery object.

In this particular example you can see them being created jQuery().find( selector ) and jQuery( elem || [] )

answered Dec 08 '11 at 13:27

Quentin

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score 2 · Answer 2 · answered Dec 08 '11 at 13:33

I will try to explain with an example. For this example you should open chrome developer console or firebug.

var $m = function() {
    return this;
};
$m.log = function(param) {
    console.log(param);
    return this;
};
$m.alert = function(param) {
    this.log('alerting: ' + param);
    alert(param);
    return this;
};
$m.log('start').alert('Sample message').log('end').log('success');

As you can see, every function returns this, which refers to $m for this example. This way I can chain as many methods as I want.

score 2 · Accepted Answer · answered Dec 08 '11 at 13:39

As per your comment:

This is from the example here: http://en.wikipedia.org/wiki/Fluent_interface#JavaScript With extra comments

var Car = function() {

        var speed, color, doors;

        this.setSpeed = function(speed) {
                this.speed = speed;
                **//Returns the reference to the calling `car` object** 
                return this; 
        };

        this.setColor = function(color) {
                this.color = color;
                **//Returns the reference to the calling `car` object** 
                return this; 
        };

        this.setDoors = function(doors) {
                this.doors = doors;
                **//Returns the reference to the calling `car` object** 
                return this; 
        };

};

// Fluent interface
**//Each method returns a reference to the object itself**
**//so the next method chain is refering back to the previous returned value**
**//ie - itself, the orginal object that started the call chain**
myCar = new Car();
myCar.setSpeed(100).setColor('blue').setDoors(5);

// Example without fluent interface
**// normal, non fluent style, where each method returns Void**
**// so you need to start with the object reference yourself each time**
myCar2 = new Car();
myCar2.setSpeed(100);
myCar2.setColor('blue');
myCar2.setDoors(5);

In your example snippet from jQuery, it's here: http://stackoverflow.com/a/8431708/426894 — asawyer, Dec 08 '11 at 13:52
@PrisonerZERO Open the jQuery source in your browser, type Ctrl+F, then type `return this`. It is happening on all the lines that get highlighted. — Dennis, Dec 08 '11 at 13:57

score 1 · Answer 4 · answered Dec 08 '11 at 13:29

My understanding is this: Jquery functions return a reference to the relevant object when they execute, which is what allows you to chain operations together. So, if you perform an operation to find a div, the function returns a reference, allowing you to immediately chain another operation to the end of it.

score 1 · Answer 5 · answered Dec 08 '11 at 14:19

Using return this; returns the parent OBJECT

var myObject = {
  a: function(){
    alert('a');
    return this; 
    //this == myObject, so returning myObject means 
    //the next function in the chain will call myObject.___
  },
  b: function(){
    alert('b');
    return this;
  }
}
myObject.a().b();//alerts 'a' then alerts 'b'

How does jQuery accomplish chaining of commands?

5 Answers5

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