This

this is the object that the function is a property of

this can also be thought of as whatever to the left of . is referring to.

In the Global context this will refer to the Window object.

Examples

Question

// What would `a()` print out
const a = function(){
    console.log(this);
    const b = function() {
        console.log(this);
        const c = {
            hi: function(){
                console.log(this);
            }
        }
        c();
    }
}
a();

Answer

// It would print out:
window // a
window // b
{hi: f() } // c

Explanation

// This scope chain 
const a = function(){
    const b = function() { }
    b();
}
a();

// is equavilent to 
window.a(b())

// When `c` is called then it looks like
window.a(b(c.hi()))

Since this is the object that the function is a property of. That means that window is the only thing that has called BOTH a and b therefore the this keyword only refers to window at this point.

When it goes down to c.hi() then c is the object that the function is a property of Therefore this will refer to c because it's the thing that has made the hi() call.