In JavaScript, is chained assignment okay?

Your colleague is right:

var a = b = [];
a.push('something');
console.log(b); // outputs ["something"]

but:

var a = [],
  b = [];
a.push('something');
console.log(b); // outputs []

With the first example b is a reference to a, and b becomes a global variable, accessible from anywhere (and it replaces any b variable that may already exist in the global scope).

Your colleague is right. The first statement creates a new, empty array. Then, a reference to this array is assigned to b. Then, the same reference (which is the result of the assignment expression) is assigned to a. So a and b refer to the same array.

In all other cases, you create two individual arrays.

By the way: This behavior is quite common and is the same in all C based programming languages. So this is not JavaScript specific.

Yes, they're not the same. var a = b = [] is equivalent to

var a;
b = [];
a = b;

Not only do both a and b get assigned the same value (a reference to the same empty array), b is not declared at all. In strict mode in ECMAScript 5 and later, this will throw a ReferenceError; otherwise, unless there is already a variable b in scope, b is silently created as a property of the global object and acts similarly to a global variable, wherever the code is, even inside a function. Which is not good.

You can see this quite easily:

(function() {
    var a = b = [];
})();

console.log(b); // Shows []