Question

Hey everyone, this is #23 from John Resig Advanced JavaScript http://ejohn.org/apps/learn/#23, called

What happens if a function is an
object property.

1) regarding vocabulary, the variable katana is the object, right? If the anonymous function is its property, then what is “use” called? I thought “use” would have also been called a property? or is “use” also an object because it contains a value, namely a function?

2). Is the purpose of the function to change isSharp: true to isSharp: false? What does !this.isSharp exactly do?

3) When it asserts !katana.isSharp, what is it actually asserting? that isSharp has now been set to “false”?

var katana = {
  isSharp: true,
  use: function(){
    this.isSharp = !this.isSharp;
  }
};
katana.use();
assert( !katana.isSharp, "Verify the value of isSharp has been changed." );

Answer 1

1. Yes, katana is an object (created using the { ... } notation). “use” is the name of the property of the object whose value will be the anonymous function (which is also an object).

2. The function inverts the value of isSharp (so from true to false or false to true).

3. It is asserting that isSharp is something which does not evaluate to true (this is nearly everything except undefined, null, false, 0, etc). In this case, since isSharp is always either true or false, it is asserting that it is false.

The main point (and cool part) of the sample is this line:

katana.use();

This first fetches the value of the “use” property from the katana object (that’s the katana.use part). The value is the anonymous function from before. Then, that function is executed (that’s the () part). The really cool part is that it is executed on behalf of the katana object — that means this in the anonymous function is a reference to the katana object when it’s called that way.