Javascript comma syntax

Question

Why do so many developers write commas this way?

var npm = module.exports = new EventEmitter
  , config = require("./lib/config")
  , set = require("./lib/utils/set");

Not this way?

var npm = module.exports = new EventEmitter,
    config = require("./lib/config"),
    set = require("./lib/utils/set");

Answer 1

They write them with the "," at the beginning of the line to make it easier to maintain the code (add lines or remove/comment out lines).

Given this:

var npm = module.exports = new EventEmitter
  , config = require("./lib/config")
  , set = require("./lib/utils/set");

It's much cleaner and easier to do this:

var npm = module.exports = new EventEmitter
//  , config = require("./lib/config")
  , set = require("./lib/utils/set");

as well as add new lines like this:

var npm = module.exports = new EventEmitter
  , config = require("./lib/config")
  , anothervalue = require("./lib/aval")
  , anothervalue2 = require("./lib/aval2")
  , set = require("./lib/utils/set");

Answer 2

I never saw that pattern before in JS, so I'm not sure if there are that many developers that use it, but I would guess they do that so variable names are aligned, to emphasize that var actually defines three variables in your sample code.

If that's the case however, it would be clearer (and less weird) to just use var three times:

var npm = module.exports = new EventEmitter;
var config = require("./lib/config");
var set = require("./lib/utils/set");

Answer 3

This is strictly a programmer-specific syntaxtual preference. I know a lot of DBA's that sem to prefer that method, while I prefer the trailing comma myself. there's nor real difference except for personal preference/education.