Context problem

[buchuitoudegou]

function func1() {
    console.log(this.a);
}

function func2() {
    this.a = 'a';
    func1();
}

func2();
console.log(this.a)

output:

a
undefined

why func1 and func2 share the same context?

[buchuitoudegou]

context probably means the value of this in the same scope. If a function defined in a scope, the value of this will be global when it's called directly. In other word, the context of the function is global unless call, apply or new are used or called by other object (e.g. obj.func()).

[buchuitoudegou]

Furthermore, if call, apply or new are used to call a function, the context of the function will be changed while the context of the functions defined in it will not.

function a() {
    console.log(this.b);
    const that = this;
    function b() {
        console.log(this === that);
    }
    b();
}
a.call({ b: 'b' })
a();

output:

b
false
undefined
true

[buchuitoudegou]

When a function is called, 3 steps are executed:

1. create variables and activate objects
2. create scope of the function
3. set the value of this