What does a comma do in assignment statements in JavaScript?

Question

I found this in a piece of code and i\'m wondering what it does? Assign b to x... but what\'s with the ,c?

var x = b, c;

Answer 1

c is undefined.

This is equivalent:

var x = b;
var c;

Answer 2

That declares two variables, x and c, and assigns value b to variable x.

This is equivalent to the more explicit form^*:

var x = b;
var c;

JavaScript allows multiple declarations per var keyword – each new variable is separated by a comma. This is the style suggested by JSLint, which instructs developers to use a single var per function (the error message from JSLint is Combine this with the previous 'var' statement.).

^{* Actually, due to hoisting it will be interpreted as var x; var c; x = b.}

Answer 3

It's the same as

var x = b;
var c; 

One of those so clever it's extremely stupid additions to a language.

Answer 4

That defines two local variables x and c - while setting x's value equal to the value of b.