How to Preserve `this` Scope When Using Instance Methods as Callbacks in JavaScript?

this Conundrum in JavaScript Callbacks

When assigning instance methods as event handlers, the scope of this shifts from the object instance to the event caller. This can present a problem and requires the use of a workaround like this:

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

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

While functional, this approach may seem awkward. Is there a better way?

Understanding the Closure

The underlying issue goes beyond jQuery, rooted in the behavior of closures in JavaScript. Closures allow inner functions to access variables declared in their outer scopes. However, this is a special variable that can dynamically change its scope.

To channel this into embedded functions, one can assign it to an alias, such as abc:

var abc = this;

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 allows abc to retain the correct object reference even within embedded functions.

Conclusion

Aliasing this enables efficient use of instance methods as callbacks without compromising the scope. This technique applies not only to this but also to other pseudo variables like arguments.

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