As discussed previously, when a function is defined, it is attached to its enclosing scope via closure. Scope is the set of rules that controls how references to variables are resolved.

But functions also have another characteristic besides their scope that influences what they can access. This characteristic is best described as an execution context, and it’s exposed to the function via its this keyword.

Scope is static and contains a fixed set of variables available at the moment and location you define a function, but a function’s execution context is dynamic, entirely dependent on how it is called (regardless of where it is defined or even called from).

this is not a fixed characteristic of a function based on the function’s definition, but rather a dynamic characteristic that’s determined each time the function is called.

The outer classroom(..) function makes no reference to a this keyword, so it’s just like any other function we’ve seen so far. But the inner study() function does reference this, which makes it a this-aware function. In other words, it’s a function that is dependent on its execution context.

The inner study() function returned by classroom("Kyle") is assigned to a variable called assignment. So how can assignment() (aka study()) be called?

In this snippet, we call assignment() as a plain, normal function, without providing it any execution context.

Since this program is not in strict mode (see Chapter 1, “Strictly Speaking”), context-aware functions that are called without any context specified default the context to the global object (window in the browser). As there is no global variable named topic (and thus no such property on the global object), this.topic resolves to undefined.

A copy of the assignment function reference is set as a property on the object, and then it’s called as homework.assignment(). That means the this for that function call will be the homework object. Hence, this.topic resolves to "JS".

Lastly:

A third way to invoke a function is with the call(..) method, which takes an object (otherHomework here) to use for setting the this reference for the function call. The property reference this.topic resolves to "Math".

The same context-aware function invoked three different ways, gives different answers each time for what object this will reference.