Problem

But a can be assigned the higher of the two numbers as well. The sort method could call the function with 5 and 4 for a and b respectively. So in this case a has been assigned the higher of the two numbers. Maybe I'm missing what you're saying.

It's the return value of the function which tells the sort method which element should come first. So in this case it would be return 5 - 4; which is a positive value. This tells the sort method that the second argument it passed into the function should actually come before the first argument it passed in.

it might help to write out the compare function in what I would call long form.

the short form:

function (a, b) {
    return a - b;
}

the longer form:

function (a, b) {
    if (a < b) {
       return -1; // return some negative number, any negative number.  This tells the sort method that a should come before b.
    }
    if (a > b) {
       return 1; // return some positive number, any positive number.  This tells the sort method that b should come before a.
    }

    return 0; // They must be equal and they should remain unchanged with respect to themselves but sorted with respect to the other elements
}

Both of these functions will do the exact same thing. I'd much rather write the first one though. The return statements in the second function do not use the variables a and b at all.

So it's not that because a is first in the return statement that makes it sort in ascending order it's just that it needs to be first to make that calculation return the proper positive, negative, and zero values.

There is no subtraction. In the "function(a,b)", "a" is assigned the lowest value in the array, and "b" is assigned the highest value in the array. Then the "return a-b" instructs to organize from "a" (lowest value) to "b" (highest value). If you were to switch the "return" to "return b-a", then it would switch "b" (highest value) and "a" (lowest value), and sort them from highest to lowest. At least I'm pretty sure that's how it works.

Well, yeah, I figured that it didn't already know which one was the lowest and highest, but as far as the sorting of it goes, the "a" and "b" are assigned whichever happen to be the lowest and highest, respectively; which is why it depends on which of those variables comes first in the "return" statement.

But I do get what you are saying, so thanks for clarifying! :)