# Formatting
Before addressing optimizations, let's address some formatting problems

## `while` vs. `for` loops
It is preferred to use a `for` loop when you:

1. Have an incrementer
2. The incrementer is not used outside of the loop
3. The incrementer is either strictly increasing or strictly decreasing

This is the case with both of your while loops. They can be refactored to the following:

In `listFactors()`:

    for (var i = 2; i <= number; i++){
        if(isFactor(number, i)){
            factors.unshift(i);
        }
    }
 
In `GCF()`:

    for (var count = 0; count < factorsOfFirst.length; count++){
        var toTest = factorsOfFirst[count];
        var passTest = factorsOfEach.every(arrayContains);
        if (passTest) {
            return toTest;
        }
    }

## Avoid explicit booleans when possible
Instead of:

    if(num % fact === 0){
        return true;
    }
    else{
        return false;
    }

it is preferable to write:

    return num % fact === 0;

and the same for your other functions.

## Avoid global variables
As you correctly pointed out, you should try not to use mutable global variables; it makes the code much harder to understand. To avoid this, you could make the `arrayContains` function merely an [arrow function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions) and pass in `toTest` as well:

    var testPassed = factorsOfEach.every(arr => arr.indexOf(toTest) !== -1);

# A Cleaner Solution
Using the [Euclidean Algorithm](https://en.wikipedia.org/wiki/Euclidean_algorith) to find the GCD/GCF of two numbers (assuming non-negative numbers) is a much more cleaner and optimized approach:

    function GCF(a, b) {
        if (b === 0) return a;
        else         return GCF(b, a % b);
    }

Then, just use a [reduce](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce?v=control) statement to apply GCF to all of the numbers in the array:
 
    function findGCFofList(list) {
        return list.reduce(GCF);
    }