XMLHttpRequest inside an object: how to keep the reference to "this"

Question

I make some Ajax calls from inside a javascript object.:

myObject.prototye = {
  ajax: function() {
    this.foo = 1;

    var req = new XMLHttpRequest();
    req.open('GET', url, true);
    req.onreadystatechange = function (aEvt) {  
      if (req.readyState == 4) {  
        if(req.status == 200)  {
          alert(this.foo); // reference to this is lost
        }
      }
  }
};

Inside the onreadystatechange function, this does not refer to the main object anymore, so I don't have access to this.foo. Ho can I keep the reference to the main object inside XMLHttpRequest events?

Answer 1

The most simple approach is usually to store the value of this on a local variable:

myObject.prototype = {
  ajax: function (url) { // (url argument missing ?)
    var instance = this; // <-- store reference to the `this` value
    this.foo = 1;

    var req = new XMLHttpRequest();
    req.open('GET', url, true);
    req.onreadystatechange = function (aEvt) {  
      if (req.readyState == 4) {  
        if (req.status == 200)  {
          alert(instance.foo); // <-- use the reference
        }
      }
    };
  }
};

I suspect also that your myObject identifier is really a constructor function (you are assigning a prototype property).

If that's the case don't forget to include the right constructor property (since you are replacing the entire prototype), which is simply a reference back to the constructor.

Maybe off-topic to this issue but recommended to read:

• Constructors considered mildly confusing

Answer 2

An other simple solution is to bind your onreadystatechange function to this. bind -ing the function does essentially the same thing as in CMS's answer (that is, adding the value to the closure), but bind does it in a transparent way: you keep using this instead of setting a instance variable.

Here is a Function#bind implementation if your code base doesn't include one :

Function.prototype.bind = function(obj) {
    var __method = this;
    var args = []; for(var i=1; i<arguments.length; i++) args.push(arguments[i]);
    return function() {
        var args2 = []; for(var i=0; i<arguments.length; i++) args2.push(arguments[i]);
        return __method.apply(obj, args.concat(args2));
    };
}

And here is how you can use it in your code :

myObject.prototype = {
  ajax: function() {
    this.foo = 1;

    var req = new XMLHttpRequest();
    req.open('GET', url, true);
    req.onreadystatechange = function (aEvt) {  
      if (req.readyState == 4) {  
        if(req.status == 200)  {
          alert(this.foo); // reference to this is *kept*
        }
      }
    }.bind(this) // <- only change
  }
};