.

If the tests don't catch it, don't blame the solution.

Your squeakyClean function should accept an input array of values and return a new array with all empty strings, 0, null and undefined removed.

It clearly states what to return.

the function of the example of the instruction is adder, I think it should keep all the return values in that case.
but it's not a big deal, not the key point of this kata.

It's normal practise use [].someFn .call()/.apply(). If you like save memory, why you chose JS?

Actually the kata description is vague about it.

.returned(val) — returns true if spy returned val, else returns false

If I call my spying function and it returned 1, I can say, "Yes, it returned 1". The description does not mention some kind of memory, neither the tests.

You've lost the last word dolor.

In most cases you don't want to do that. If function relay on this in its body (which is equal to window unless function defined in object or "use strict"; is used) you probably will encounter some side effects :) It's safer to pass null and use bind if you need to keep function's current context.

Perhaps an example will make it clear:

var obj = {
  val: 0,
  fn: function func() {
    this.val++;
    return this.val;
  }
}
window.val = 100;
obj.fn.val = 1000;

obj.fn(); // 1 -> this === obj
obj.fn.apply(obj); // 2
obj.fn.apply(null); // 101
obj.fn.apply(obj.fn); // 1001

var fn = obj.fn;
// this now refer to window instance
fn(); // 102
fn.apply(null); // 103 -> same as fn()
fn.apply(fn); // 1002 -> don't expected behaviour
fn.apply(obj); // 3

var bindedFn = obj.fn.bind(obj);
// this now is always refer to obj
bindedFn.apply(obj); // 4
bindedFn.apply(null); // 5
bindedFn.apply(obj.fn); // 6