Chaining assignments as part of a var declaration will create global variables unintentionally.

For example:

(function foo() {    
    var a = b = 0;
})()
console.log('a: ' + a);
console.log('b: ' + b);

Will result in:

Uncaught ReferenceError: a is not defined
'b: 0'

In the above example, a is local but b becomes global. This is because of the right to left evaluation of the = operator. So the above code actually evaluated as

var a = (b = 0);

The correct way to chain var assignments is:

var a, b;
a = b = 0;

Or:

var a = 0, b = a;

This will make sure that both a and b will be local variables.