Why this code return undefined?

Question

I am new to javascript, why this code return undefined?

    const foo = {  
      bar: function() { return this.baz; },
      baz: 1,
    };

    console.log((function() {
      return typeof arguments[0]();
    })(foo.bar));

Where did you find this code? That IIFE and `arguments[0]` thing looks unnecessarily complicated and irrelevant to your question. — Bergi, Jun 15 '17 at 04:27
When you are executing arguments[0]() `this` refers to the context it was called in, in this case, it would most likely be `window` and `window` won't have a property called `baz` — JohanP, Jun 15 '17 at 04:29
@Tushar Are there? Feel free to reopen and answer, but I think the only/main problem is the misunderstanding of the [`this` keyword](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this) — Bergi, Jun 15 '17 at 04:30
@JohanP Actually `this` will refer to the `arguments` of the IIFE, but yes it's just weird. — Bergi, Jun 15 '17 at 04:30

Prakash N · Answer 1 · 2017-06-15T04:59:29.100

-1

If you are expecting an answer of number that can not be returned from you code. This is a misunderstanding of this keyword (as pointed out by @Tushar @Bergi)

The following code does it.

function Foo(){
    this.baz = 1;
};

Foo.prototype.bar = function() {
    "use strict";
    return this.baz;
}

var foo = new Foo();
console.log((function() {
    "use strict";
    return typeof arguments[0];
})(foo.bar()));

edited Jun 15 '17 at 04:59

answered Jun 15 '17 at 04:27

Prakash N

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The problem is with calling the `bar` function. Not just show the typeof that variable. – Tushar Jun 15 '17 at 04:31
What do you think is the "correct result" and why? I'm pretty certain that the OP expected `"number"`, the typeof `1`, which is not what this code yields. – Bergi Jun 15 '17 at 04:32
Edited the code for yeilding `number`. – Prakash N Jun 15 '17 at 05:02
Yes, you moved the call, but why did you change `foo` to be an instance with a prototype?! – Bergi Jun 15 '17 at 05:03
Is there any other way to do this? (apart from creating an instance) @Bergi – Prakash N Jun 15 '17 at 05:10
How can you consume the use of `this` keyword without creating an instance? – Prakash N Jun 15 '17 at 05:11
The original object literal would have worked just fine. What do you mean by "*consume the use of `this`*"? – Bergi Jun 15 '17 at 05:15
Can you please edit the answer or suggest a new answer to the one you are referring to? – Prakash N Jun 15 '17 at 05:17

Why this code return undefined?

1 Answers1