Multiple left-hand assignment with JavaScript

Actually,

var var1 = 1, var2 = 1, var3 = 1;

is not equivalent to:

var var1 = var2 = var3 = 1;

The difference is in scoping:

function good() {
  var var1 = 1, var2 = 1, var3 = 1;
}

function bad() {
  var var1 = var2 = var3 = 1;
}

good();
console.log(window.var2); // undefined

bad();
console.log(window.var2); // 1. Aggh!

Expand snippet

Actually this shows that assignment are right associative. The bad example is equivalent to:

var var1 = (window.var2 = (window.var3 = 1));