In Javascript - what do multiple commas do in a var statement [duplicate]

Question

I have been looking through new wordpress theme twentyfourteen and notice the following line in their javascript file:

var nav = $( '#primary-navigation' ), button, menu;

Can somebody explain me what does it mean, it does not look like a multiple variable assignment in a single line. Also button and menu are not defined yet, so how come it does not produce an error?

Answer 1

You are just declaring three variables and assigning value to only nav. Both button and menu will have undefined, by default.

var a = 1, b, c;
console.log(a, b, c);

Output

1 undefined undefined

Instead, if you see something like this

var d = (1, 2, 3);
console.log(d);

Output

3

The expressions within the brackets will be evaluated from left to right and the result of the last evaluation will be assigned to d.