Why does the { position affects this Javascript code?

Question

I spent a fair bit of time on this Javascript issue (you can tell I am a JS noob):

Take some well written Javascript code like this example of the Revealing Module Pattern:

Running it works fine. Then move the "{" to the next line (as a C# developer I set up all my environments to put curly braces on new lines) and run it again.

  return   
  {
    someMethod : myMethod,
    someOtherMethod : myOtherMethod
  };

It now gets quite a few JS errors around "13 Line breaking error 'return'." and "Uncaught SyntaxError: Unexpected token : " in Chrome Debugger.

My question is, how can something syntactically affect the Javascript like this?

I have set it up here in JSFiddle (to get it to work, move the { after "return" back on to the same line)

Answer 1

One of JavaScript’s worst features is Automatic Semicolon Insertion.

return; // a semicolon is implicitly inserted here

And this part is almost valid JavaScript, but not quite, so you get a syntax error:

{
    someMethod : myMethod,
    someOtherMethod : myOtherMethod
};

If you had tried to do this:

return
    [ 1, 2, 3,
      4, 5, 6,
      7, 8, 9 ];

it would have just returned undefined all the time, and that would have been bad. ASI sucks, but we’re stuck with it now, especially since semicolonless code has become a fad.

What does this do?

return a
     + b
     + c;

This?

return e
     / f /g;

Okay, okay, maybe that one’s a bit contrived, and maybe this isn’t entirely topical. But ASI is bad. I hope everyone gets that.

Answer 2

Because the ECMA standard section 12.9 states you can't have a new line between the return keyword and its expression.

    ReturnStatement :
       return ;
       return [no LineTerminator here] Expression ;

Answer 3

Javascript does something called Automatic semi-colon insertion which I believe is what is affecting your results here. Basically, it sees the return statement with nothing after on the line, and thinks that's the end of the line and returns, ending the function.