Alex Rivera | Logout

What is the rationale for the behavior of the 'this' keyword in JavaScript?

Asked 2009-02-12T13:14:30.537
9

I am asking this from a language design point of view. So I am trying to find out

  1. What is the rationale for the behavior of this?
  2. To what degree the behavior of this was a mistake, or could be improved upon?

To clarify why I'm uneasy about this, consider this example:

var a = {};
a.f = function(){ return this; }
var f = a.f;
// f() != a.f()

Note how easily the object to which f() belong is lost: separated from a, this becomes the global object (window for browsers).

Now consider:

var newA = function(){
    var self = {};
    self.f = function(){ return self; }
    return self;
}

var a = newA();
var f = a.f;
// f() == a.f() !

Without using this at all, we are able to establish and maintain the object context regardless of where or how the method is used. I can't help but think that, with the power that closures provide, this becomes superfluous, and perhaps even a little dangerous...

I'm not on some vendetta against this, or looking to start an argument; I'm merely trying to better understand it. I do appreciate that 'this' can be useful, but recognize that it can be confusing as well... Certainly confusing to beginners, and perhaps to experts as well in sufficiently obscure cases.

And yet, it remains a heavily-used and seemingly well-respected part of the language, in a time when other core aspects of the language seem fair game for shunning (i.e., Crockford and with or new). What am I missing then, that makes this indispensable?

javascript language-design
Edit
Report

1 Answer

8

I don't think making "this" unbound was a mistake. It can sometimes be confusing at first, but there are good reasons for the way it is. The first one that comes to mind is that, since JavaScript is not a class based language, functions are not associated with any specific class, so there's not a consistent way to automatically bind "this" to the correct object instance. For example,

function Person(first, last, age) {
    this.firstName = first;
    this.lastName = last;
    this.age = age;
}

Person.prototype.getFullName = function() {
    return this.firstName + " " + this.lastName;
};

"this" needs to refer to a Person object, but the function assigned to Person.prototype.getName doesn't have any way of knowing how it's going to be used, so "this" needs to be bound to whatever object it is called on.

Where this causes a problem, is when you have nested functions.

// This is a really contrived example, but I can't think of anything better
Person.prototype.getInfo = function() {
    // get name as "Last, First"
    function getNameLastFirst() {
        // oops. "this" is the global object, *not* the Person
        return this.lastName + ", " + this.firstName;
    }

    // expect something like "Crumley, Matthew: Age 25",
    // but you get "undefined, undefined: Age 25"
    return getNameLastFirst() + ": Age " + this.age;
};

The syntax artificialidiot suggested would be convenient, but it's pretty easy to bind "this" to a specific object using apply:

function bind(func, obj) {
    return function() {
        return func.apply(obj, arguments);
    };
}

Person.prototype.getInfo = function() {
    // get name as "Last, First"
    var getNameLastFirst = bind(function () {
        return this.lastName + ", " + this.firstName;
    }, this);
answered 2009-02-12T15:50:47.600

Your Answer