Why this function return undefined?

Question

I don't undetstand why the following function doesn't return {bar:"hello"} and instead it returns undefined.

function foo2() {
    return
    {
        bar: "hello"
    };
}

There's a carriage return after `return` and before the opening brace. Try `return {` instead. — sherb, Dec 05 '14 at 05:30
You might want to read up on [automatic semicolon insertion](http://stackoverflow.com/questions/2846283/what-are-the-rules-for-javascripts-automatic-semicolon-insertion-asi) in JavaScript. — Ben, Dec 05 '14 at 05:31

score 5 · Accepted Answer · answered Dec 05 '14 at 05:30

5

That's because its compiled to the below because of javascript's auto semi-colon insertion.

function foo2() {
    return; // notice the semi-colon here?
    {
        bar: "hello"
    };
}

And since return; is called, the function terminates without going to the next line of code. To make it work correctly just put the opening bracket right after return as in return {

You'd be better off by using semi-colons than omitting them. Want reasons? check out Dangers of Javascript's automatic semicolon insertion

answered Dec 05 '14 at 05:30

Amit Joki

58,320
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Note that this is the *exact* example for the dangers of ASI that is shown in pretty much every JS tutorial, styleguide, book, etc. ever created in the history of JS. Anybody who has spent even 3 seconds learning JS knows this. – Jörg W Mittag Dec 05 '14 at 05:32
Well, _maybe_ four seconds... :-) – sherb Dec 06 '14 at 00:08

Leo · Answer 2 · 2014-12-05T08:01:37.947

1

A semicolon is inserted after return by JS engine.

function foo2() {
    return;
    {
        bar: "hello"
    };
}

Change to this is ok

function foo2() {
    return {
        bar: "hello"
    };
}

About auto semicolon insertion, aka ASI, you may want to read this, this and this.

edited Dec 05 '14 at 08:01

answered Dec 05 '14 at 05:30

Leo

13,428
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Why this function return undefined?

2 Answers2