var self = this?

Question

Using instance methods as callbacks for event handlers changes the scope of this from "My instance" to "Whatever just called the callback". So my code looks like this

function MyObject() {
  this.doSomething = function() {
    ...
  }

  var self = this
  $('#foobar').bind('click', function(){
    self.doSomethng()
    // this.doSomething() would not work here
  })
}

It works, but is that the best way to do it? It looks strange to me.

Answer 1

Yeah, this appears to be a common standard. Some coders use self, others use me. It's used as a reference back to the "real" object as opposed to the event.

It's something that took me a little while to really get, it does look odd at first.

I usually do this right at the top of my object (excuse my demo code - it's more conceptual than anything else and isn't a lesson on excellent coding technique):

function MyObject(){
  var me = this;

  //Events
  Click = onClick; //Allows user to override onClick event with their own

  //Event Handlers
  onClick = function(args){
    me.MyProperty = args; //Reference me, referencing this refers to onClick
    ...
    //Do other stuff
  }
}

Answer 2

var functionX = function() {
  var self = this;
  var functionY = function(y) {
    // If we call "this" in here, we get a reference to functionY,
    // but if we call "self" (defined earlier), we get a reference to function X.
  }
}

edit: in spite of, nested functions within an object takes on the global window object rather than the surrounding object.

Answer 3

I think it actually depends on what are you going to do inside your doSomething function. If you are going to access MyObject properties using this keyword then you have to use that. But I think that the following code fragment will also work if you are not doing any special things using object(MyObject) properties.

function doSomething(){
  .........
}

$("#foobar").ready('click', function(){

});

Answer 4

Just adding to this that in ES6 because of arrow functions you shouldn't need to do this because they capture the this value.

Answer 5

One solution to this is to bind all your callback to your object with javascript's bind method.

You can do this with a named method,

function MyNamedMethod() {
  // You can now call methods on "this" here 
}

doCallBack(MyNamedMethod.bind(this)); 

Or with an anonymous callback

doCallBack(function () {
  // You can now call methods on "this" here
}.bind(this));

Doing these instead of resorting to var self = this shows you understand how the binding of this behaves in javascript and doesn't rely on a closure reference.

Also, the fat arrow operator in ES6 basically is the same a calling .bind(this) on an anonymous function:

doCallback( () => {
  // You can reference "this" here now
});

Answer 6

This question is not specific to jQuery, but specific to JavaScript in general. The core problem is how to "channel" a variable in embedded functions. This is the example:

var abc = 1; // we want to use this variable in embedded functions

function xyz(){
  console.log(abc); // it is available here!
  function qwe(){
    console.log(abc); // it is available here too!
  }
  ...
};

This technique relies on using a closure. But it doesn't work with this because this is a pseudo variable that may change from scope to scope dynamically:

// we want to use "this" variable in embedded functions

function xyz(){
  // "this" is different here!
  console.log(this); // not what we wanted!
  function qwe(){
    // "this" is different here too!
    console.log(this); // not what we wanted!
  }
  ...
};

What can we do? Assign it to some variable and use it through the alias:

var abc = this; // we want to use this variable in embedded functions

function xyz(){
  // "this" is different here! --- but we don't care!
  console.log(abc); // now it is the right object!
  function qwe(){
    // "this" is different here too! --- but we don't care!
    console.log(abc); // it is the right object here too!
  }
  ...
};

this is not unique in this respect: arguments is the other pseudo variable that should be treated the same way — by aliasing.