I just finished reading an article about ECMAScript 5 strict mode.

It says that ECMAScript 5 added .bind().

var obj = {
  method: function(name){
    this.name = name;
  }
};

obj.method.bind(obj,"hello");

Isn't it identical to obj.method.call(obj,"hello") ??

I just finished reading an article about ECMAScript 5 strict mode.

It says that ECMAScript 5 added .bind().

var obj = {
  method: function(name){
    this.name = name;
  }
};

obj.method.bind(obj,"hello");

Isn't it identical to obj.method.call(obj,"hello") ??

• javascript
• call
• bind
• ecmascript-5

Share Improve this question Follow edited Mar 9, 2012 at 15:20 kangax 39.2k1313 gold badges100100 silver badges135135 bronze badges asked Mar 8, 2012 at 21:54 MoonMoon 22.6k7272 gold badges198198 silver badges276276 bronze badges 1

• 1 obj.method.bind(obj, "hello")() would be identical. – pimvdb Commented Mar 9, 2012 at 15:24

Add a ment  | 

2 Answers 2

Sorted by: Reset to default 12

No, it's not identical.

With bind you're producing a function, without calling anything. With call — as in your obj.method.call(obj, 'hello') — you're actually calling a method.

An "identical" expression to obj.method.bind(obj, 'hello') would be function(){obj.method.call(obj, 'hello')}. That's more cruft. And that's the cruft ES5 is trying to provide convenience for.

There are also historical reasons for introduction of bind; it first became popular as one of the helper methods in Prototype.js few years ago. Then made its way to other popular libraries, such as underscore.js. ES5 just followed what was already popular and in demand.

The main reason of creating a new function out of existing one is to be able to pass it together with the object on which it will be invoked. Consider this:

var obj = {
  x:"Hello",
  printPropX: function() {
    alert(this.x);
  }
};

setTimeout(obj.printPropX.bind(obj), 1000); // will alert "Hello" after 1 second

setTimeout(obj.printPropX, 1000); // will fail as 'this' won't be defined inside the function body once invoked

Although printPropX() is a member function of object 'obj', it won't be invoked with 'this' set to 'obj', unless bound with bind().