Why do Javascript functions return "undefined" on creation?

Question

I understand the difference between undefined and null as a type; I'm just not grasping the following event:

function x() {
    return 0;
}

undefined

Why does this happen? What is going on under the hood that is causing the environment to return an "undefined" value?

UPDATE: I'm not invoking the function in the example above, just defining it and the console returns this "undefined" value.

When you call this function, it return undefined instead of 0 ? — Léo R.
– Léo R., Commented Feb 13, 2018 at 10:14
You should call/console the function as console.log( x() ); and not as console.log( x ); . I am just assuming here :) — Eddie
– Eddie, Commented Feb 13, 2018 at 10:16
I think he refers to the console. When you just declare the function. — Zoltán Jére
– Zoltán Jére, Commented Feb 13, 2018 at 10:16
@eisbehr I think he tried this in console and means at the time of declaration — ata
– ata, Commented Feb 13, 2018 at 10:16

Mathias Dolidon · Accepted Answer · 2018-02-13 10:27:15Z

4

This fuction definition is a statement. In JS, statements don't return values, which, in JS, is an undefined value.

In JS, assignments with var are statements too, but assignments without var behave as expressions : their whole value is the value being assigned.

Therefore, in the console :

> x=function() {return 0;}
< ƒ () {return 0;}

edited Feb 13, 2018 at 10:27

answered Feb 13, 2018 at 10:22

Mathias Dolidon

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3 Comments

Charles Saag Over a year ago

Okay, thank you. So if I understand correctly, Javascript always wants a return from every statement and command?

Mathias Dolidon Over a year ago

Yes you can see it as such : in JS a statement can be used in place of an expression, and its value will be undefined (which makes it pretty useless in that place).

Stephan Luis Over a year ago

undefined is the implicit return type in js, see stackoverflow.com/a/20915520/2455159

Quentin · Accepted Answer · 2018-02-13 10:19:06Z

2

What is going on under the hood that is causing the environment to return an "undefined" value?

In this case, undefined represents the lack of a value.

You haven't run any expression, so there is no value to arise from it.

answered Feb 13, 2018 at 10:19

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