Home Javascript OOP : Explain why f.a returns something whereas f.b and c.a returns undefined

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Javascript OOP : Explain why f.a returns something whereas f.b and c.a returns undefined

July 9, 2022

Explain why f.a returns something whereas f.b and c.a returns undefined. Don’t answer : because it’s like that or you don’t really understand javascript and learn by rotte as Feynman says 😉

function f() {
  f.a = "test 1";
  this.b = "test 2";
  return this;
}

let c = f();
console.log(f.a);
console.log(f.b);
console.log(c.a);

>Solution :

After you call f, it gives itself (f the Function object) a property of a assigned to the string "test 1".

Because f is not called with new, this inside the function body refers to the global object (window in the browser, global in node). This means the assignment to this.b inside the function body is equivalent to window.b = "test 2".

Then the function f returns this, which is the global object, so c = global.

The global object hasn’t been assigned a property a, so c.a is undefined. But c.b would be "test 2"