CODEWITHSUNDEEP
javascript
SeasonHuang
SeasonHuang

Reputation: 157

A case about javascript closure and anonymous function

Let see code first

var name = "The Window"; 
var object = { 
  name : "My Object", 
  getNameFunc : function(){ 
    return function(){ 
      return this.name; 
    }; 
   } 
}; 

alert(object.getNameFunc()());

the result is "The Window"; I want to know what happend in every step. I think this is point to the object which call this function; right? But why in this case this is window

Upvotes: 1

Views: 70

Answers (2)

I Hate Lazy
I Hate Lazy

Reputation: 48761

The second function invocation doesn't take place from the context of an object, but rather from the function returned from the first invocation.

Because there's no object context, the this value becomes the default window just like any other function.

 //   v-----------------v------function has context
alert(object.getNameFunc()());
            // -----------^----second function was returned from the first
            //                   and invoked so there's no object context

If we instead assigned the returned function to object, then invoked it from that context, this would then be a reference to object.

obj.foo = object.getNameFunc();
obj.foo();  // "My Object"

Same exact function, but now it's being invoked from as a property of object, which implicitly sets its this value to object.

The rules of this are really pretty simple and easy to understand, but may not be what you'd expect at first.

The this value is very dynamic, and is based entirely on how a function is invoked.

foo();            // 'this' is 'window'

object.foo();     // 'this' is 'object'

foo.call(object); // 'this' is 'object'
foo.apply(object);// 'this' is 'object'

var bar = foo.bind(object);
bar();            // 'this' is 'object'

So you can see that the default is window when the function is invoked without any sort of connection to another object.

But when we call the function as a property of an object, this suddenly refers to that object.

Or we can use .call, .apply or .bind to manually set the this value of the function call.

Upvotes: 1

Niet the Dark Absol
Niet the Dark Absol

Reputation: 324650

Since it's a new closure, the meaning of this is lost. this has always been a tricky thing to deal with, so I prefer to always start such a function with var that = this to ensure that the meaning is not lost within closures.

Upvotes: 1

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