In JavaScript, is chained assignment okay?

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 []