Question

I have 2 blocks of code, one that does not work, and one that works because I assign that = this and use that in my function instead of this. Can someone help me understand why this is so. It would help to know how one should think of accessing variables in functions in objects in JavaScript, and the nature of “this”, if I am saying that right (if not, please enlighten me). Thank you!

var add = function (x, y) {
  return x + y;
  }

var myObject = {
  value: 0,
  increment: function (inc) {
    this.value += typeof inc === 'number' ? inc : 1;
    }
};

myObject.double2 = function () {
  // var that = this; 

  var helper = function () {
    this.value = add(this.value, this.value)
  };

  helper();
};

myObject.increment(100);
document.writeln(myObject.value); // Prints 100
myObject.double2();
document.writeln('<BR/>'); // Prints <BR/>
document.writeln(myObject.value); // Prints 100, **FAILS**

And the modified code:

var add = function (x, y) {
  return x + y;
  }

var myObject = {
  value: 0,
  increment: function (inc) {
    this.value += typeof inc === 'number' ? inc : 1;
    }
};

myObject.double2 = function () {
  var that = this;  

  var helper = function () {
    that.value = add(that.value, that.value)
  };

  helper();
};

myObject.increment(100);
document.writeln(myObject.value); // Prints 100
myObject.double2();
document.writeln('<BR/>'); // Prints <BR/>
document.writeln(myObject.value); // Prints 200 - **NOW IT WORKS**

Answer 1

The first one doesn’t work because when each function’s this depends on how it was called.

First you do myObject.double2() and so this = myObject. But inside double2, you call helper() by itself and there is no object you’re calling it under (it’s not myObject.helper()). So this defaults to the global object (or window object in a browser).

In the second example, you “capture” a reference to myObject (that=this=myObject) and so that.value=myObject.value.