Question

I just watched a video of Douglas Crockford’s presentation about his 2009 book JavaScript: The Good Parts.

In the video, he explains that the following block is dangerous because it produces silent errors:

return
{
    ok: false
};

And that it should actually be written like this (emphasising that although seemingly identical the behavioural difference is crucial):

return {
    ok: false
};

You can see his comments around 32 minutes into the video here: http://www.youtube.com/watch?v=hQVTIJBZook&feature=player_embedded#!&start=1920

I have not heard this before, and was wondering if this rule still applies or if this requirement in syntax has been overcome by JavaScript developments since this statement was made.

I found this very interesting as I have NOT been writing my code this way, and wanted to check that this information was not out of date.

Answer 1

The silent error is that undefined is returned!

Semicolons are optional in JavaScript, and therefore

return
{
    ok: false
};

is parsed as if it were

return;  // Leaves function straight away
{
    ok: false   
};

JSLint will recognize such patterns and warn about them:

lint warning: unexpected end of line; it is ambiguous whether these lines are part of the same statement

lint warning: missing semicolon

lint warning: unreachable code

lint warning: meaningless block; curly braces have no impact

This has been discussed on SO in the “Strangest language feature” question.