You Don't Know JS: this & Object Prototypes

Chapter 2. this All Makes Sense Now!

In Chapter 1, we discarded various misconceptions about this and learned instead that this is a binding made for each function invocation, based entirely on its call-site (how the function is called).

Call-Site

To understand this binding, we have to understand the call-site: the location in code where a function is called (not where it’s declared). We must inspect the call-site to answer the question: what is this this a reference to?

Finding the call-site is generally “go locate where a function is called from,” but it’s not always that easy, as certain coding patterns can obscure the true call-site.

What’s important is to think about the call-stack (the stack of functions that have been called to get us to the current moment in execution). The call-site we care about is in the invocation before the currently executing function.

Let’s demonstrate the call-stack and call-site:

function baz() {
    // call-stack is: `baz`
    // so, our call-site is in the global scope

    console.log( "baz" );
    bar(); // <-- call-site for `bar`
}

function bar() {
    // call-stack is: `baz` -> `bar`
    // so, our call-site is in `baz`

    console.log( "bar" );
    foo(); // <-- call-site for `foo`
}

function foo() {
    // call-stack is: `baz` -> `bar` -> `foo`
    // so, our call-site is in `bar`

    console.log( "foo" );
}

baz(); // <-- call-site for `baz`

Take care when analyzing code to find the actual call-site (from the call-stack), because it’s the only thing that matters for this ...