javascript对象，回调和这个关键字

Question

I'm having trouble figuring out how to correctly reference the 'right' 'this' in a jquery ajax callback.

I have a javascript class where I define the callback:

Foo.prototype.onCallback = function(response) {
  // 'this' should refer to an instance of foo in both the following cases
  this.bar(...)    
  this.hello(...)
}

outside of the class I have:

foo1 = new Foo()
myCallback = foo1.onCallback;

$.ajax({
  ...
  success: function(response) {myCallback(response); ... }
});

Right now I believe 'this' within foo1.onCallback is referring to the html element the ajax call is attached to. How do I ensure that 'this' refers to foo1? Is there a better way to do this?

Answer 1

You could use the context property of $.ajax :

var foo1 = new Foo();

$.ajax({
  context: foo1,
  success: function(response) { this.onCallback(response); }
});

...which causes this in the callback to reference your Foo object.

Answer 2

You can't do that.

You must either write:

$.ajax({
  ...
  success: function(response) { foo1.onCallback(response); ... }
});

or this:

myCallback = foo1.onCallback.bind(foo1);

Note that Function.bind() requires ECMAScript 5.

Answer 3

Use the following idiom:

myCallback = foo1.onCallback.bind(foo1);

In fact you should take advantage of first-class function support in JavaScript even further:

foo1 = new Foo()

$.ajax({
  ...
  success: foo1.myCallback.bind(foo1)
});

See also:

• If Javascript has first-class functions, why doesn't this work?

Answer 4

You could simply add the statement alert(this); to see the if it is foo1.