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Asked: 2026-05-20T09:47:06+00:00

The following program (taken from a tutorial) prints the numbers in an array in

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The following program (taken from a tutorial) prints the numbers in an array in order from lowest to highest. In this case, the result will be 2,4,5,13,31

My question relates to the paramaters “a” and “b” for the function compareNumbers. When the function is called in numArray.sort(compareNumbers) what numbers will be the parameters a and b for the function. Does it just move along the array. For example, start with a=13 and b=2? After that, does the function run again comparing a=2 and b=31? or would it next compare a=31 and b=4?

Can someone please explain how that part works and also how it manages to sort them from lowest to highest? I don`t see how the function manages to do the necessary calculations on the numbers in the array.

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

var numArray = [13,2,31,4,5];
alert(numArray.sort(compareNumbers));
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1 Answer

  1. Editorial Team

    The particular pairs that get passed in depend on the sorting algorithm being used. As the algorithm tries to go about sorting the range, it needs to be able to compare pairs of values to determine their ordering. Whenever this happens, it will call your function to get that comparison.

    Because of this, without inside knowledge about how the sorting algorithm works, you cannot predict what pairs will get compared. The choice of algorithm will directly determine what elements get compared and in what order.

    Interestingly, though, you can actually use the comparison function to visualize how the sort works or to reverse-engineer the sorting algorithm! The website sortviz.org has many visualizations of sorting algorithms generated by passing custom comparators into sorting functions that track the positions of each element. If you take a look, you can see how differently each algorithm moves its elements around.

    Even more interestingly, you can use comparison functions as offensive weapons! Some sorting algorithms, namely quicksort, have particular inputs that can cause them to run much more slowly than usual. In “A Killer Adversary for Quicksort,” the author details how to use a custom comparator to deliberate construct a bad input for a sorting algorithm.

    Hope this helps!

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