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I apologize for posting a duplicate-looking question, (I know, that there is a bunch of similar titled questions here), but none of the questions already present seems to suit my case.

In short, what does the colon do here:

<script>
  'use strict';
  foo: 1;

  //whatever else
</script>

I supposed this to be a syntax error, but it's not. And it's not a label, I think, since adding a line break foo; throws Uncaught SyntaxError: Undefined label 'foo' (though a doc page suggests exactly this, that it's a label).

I suppose this is some recent addition to the JavaScript syntax, since I have never heard of such use of the colon.

If anyone wonders, why I'm asking this, this is my explanation: I was reading an MDN doc page and there is an example:

var func = () => { foo: 1 };               
// Calling func() returns undefined!

It shows, that the curly braces in this case are treated as block delimiters and not an object literal. So I supposed, that somehow foo: 1 on its own must be syntactically legal. And indeed it is.

There is a question, which is supposed to cover every use of the colon in JavaScript, but it doesn't mention this, and no answer there does this either.

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    Pretty sure it's a label. I suspect you're getting that error with break foo; because foo isn't a loop you can break from. Commented Oct 4, 2017 at 8:56

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If you'd have read further down the page you linked, you would see why it was written like that.

var func = () => { foo: 1 };

This is an attempt to return an object from a arrow function.

That doesn't work for reasons explained here:

This is because the code inside braces ({}) is parsed as a sequence of statements (i.e. foo is treated like a label, not a key in an object literal). (source)

So the returned value needs to be wrapped in parentheses:

var func = () => ({foo: 1});

To actually answer your question:

It's a label.

You can't just take the foo: 1 out of context like that.

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5 Comments

I actually read it further though after I posted the question. Yes probably it's a label, because Firefox doesn't throw on it. Probably it is just some Chrome specific issue.
"Yes probably it's a label" No, not "probably". It is a label. ;-) This isn't an "issue". This is working as expected.
yes, I meant this, that it throws on the break foo line and it seems illogical, at least just the error message itself. If the foo: 1 was treated like a label, so why the message is "label not found" or similar
sorry, I understood now, that the break statement is definitely expected to search for a label inside a loop or one of parent loops. So the message is also completely logical
Because the label is pointing at a single digit, 1, which makes no sense. This, on the other hand, is fine: foo:for(var i = 0; i < 10; i++){break foo}

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