javascript call() function this context

Question

In the following code,

var toString = Object.prototype.toString;

toString.call(new Date); // [object Date]
toString.call(new String); // [object String]
toString.call(Math); // [object Math]

in the call the current object is passed like ( toString.call(new Date)) , but in the receving end var toString = Object.prototype.toString; here instead of using this keyword they are using Object keyword but still got the correct answer i didn't understand this concept.Anyone pls help me in understanding this concept

Are you saying the examples above are working or aren't working? — Explosion Pills
– Explosion Pills, Commented Feb 26, 2013 at 5:55

Explosion Pills · Accepted Answer · 2013-02-26 06:00:22Z

3

.call executes a function with the provided context. The var toString = Object.prototype.toString passes the Object's toString function to a variable .. I suppose for easy access.

Then, you can use toString.call([new context here]) to call the Object's toString method with the provided context. As you can see, this prints out [object OBJECTNAME] as a string.

Object's toString is different than some others. For example:

(new Date).toString(); // "Tue Feb 26 2013 01:00:17 GMT-0500 (EST)"
(new String).toString(); // ''

answered Feb 26, 2013 at 6:00

Explosion Pills

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Comments

Aishwar · Accepted Answer · 2013-02-26 06:02:29Z

1

I am not sure I understand your question. But here goes.

var toString = Object.prototype.toString;

In this line you are reading the "prototype" property of Object and reading the "toString" property of the prototype. The toString property is a function. You stored this in your variable.

Now when you do this: toString.call(new Date);, you take that function from above and are going to apply it on the object returned by "new Date" (i.e. the this context for code in toString becomes the object returned by "new Date")

You can think of this as copying a method belonging to an object and applying it on another object.

answered Feb 26, 2013 at 6:02

Aishwar

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Comments

Matt Stone · Accepted Answer · 2013-02-26 22:29:33Z

You first need to understand how the value of this changes depending on how a function is invoked.

First up, we have member functions. Object.prototype.toString is a member function of Object.prototype. You could visualise this simply as:

Object.prototype = {
    toString: function() {} // is responsible for converting an object to it's string representation
};

When a member function is invoked, the context of this refers to the parent object (the instance of Object). As builtin JS objects extend Object, they are all able to use .toString() with varying results:

(new Object).toString(); // [object Object]
(new String).toString(); // [object String]

It helps to imagine the inner workings of .toString(), you'd imagine it might do something like:

// ..
return '[object '+ this.constructor.name + ']';
// ..

Hopefully you can now imagine how changing the value of this in the function will change the object that is being inspected.

When you invoke Object.prototype.toString via .call(), you are able to pass a new value to be used as this:

Object.prototype.toString.call(new Date); // [object Date]

I highly recommend reading Yehuda Katz' simple explanation of the 3 ways to affect the value of this when invoking a function:

http://yehudakatz.com/2011/08/11/understanding-javascript-function-invocation-and-this/

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