With separate var statements, you never end up with this situation:

    var foo = 1,
        bar = 2
        baz = 3

In 90% of the cases the code will run the same whether or not there is a comma on line 2, but in the other 10% you will get burned in odd ways now that `baz` is a global variable.

JS Lint is your friend in this case. It will log that

* There's a missing semicolon on line 2

* There's a missing semicolon on line 3

* baz was being used before it was defined.

That's just in a global context. When it's in a function you get a little more:

* the baz line is indented too far - because it's being interpreted as a new statement.

Sure, you could eyeball your code looking for this. Or just use an automated tool that'd do it for you

You can only put var at the beginning of variable declarations, but there are many other places that commas are used as a separator. So that "solution" doesn't apply in general.

I think I might have explained myself poorly. I'm suggesting using separate var statements for each assignment, e.g.:

var a = 2; var b = 4; var c = { blah: "whatever" };

Leaving out a semicolon has no effect. They can be copied without any fuss. It's easy.

Problems with semi-colons usually arise in object usage. So your solution is very specific.

    x = {[
      { id: 1, data: 0 },
      { id: 2, data: 0 }
    ],
    [
      { id: 3, data: 0 },
      { id: 4, data: 0 }
    ]}

Most people also apply this to object literals. There are no var statements in this situation.

Separate vars is what I prefer to do, but JSLint actually recommends that you change multiple var statements to a single var with variables separated by commas (and with a semicolon at the end of course!). (And also all var declarations at the top of the function...).