serena peruzzo

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Javascript is Weird Part I: Comparing Arrays

Posted at — Apr 4, 2020

Javascript is weird.

I worked in Python for the last 5 years, and in R for the 3 years before, mostly for data science applications, and I didn’t realise how much I was taking for granted until I started playing around with Javascript and spending a lot of time just wondering why!

I was pairing with Asa on Conway’s Game of Life earlier this week and ran into something weird. In our implementation, the game stops when currentGameState === nextGameState, where currentGameState and nextGameState represent respectively (surprise!) the current state of the game’s grid and the following one. Except the game didn’t stop. Ever.

It turns out, in Javascript currentGameState === nextGameState returns false even if the two arrays look exactly the same. This was counterintuitive to me but I followed Asa’s lead and we wrote a helper function that takes two arrays, a and b, as input and performs a few tests to determine if they are indeed the same. Here’s the function:

function arraysEqual(a, b) {
  if (a === b) return true;
  if (a == null || b == null) return false;
  if (a.length != b.length) return false;
  for (let i = 0; i < a.length; ++i) {
    if (typeof a[i] === "object"){
      if (arraysEqual(a[i], b[i]) == false){
        return false
    } else if (a[i] !== b[i]){
      return false
  return true;

Let’s unpack this.

Step 1: if a and b are of the same type and hold the same value return true.

Step 2. if either a or b is null return false.

Step 3. If a and b have different lengths return false.

Step 4. Loop through all elements of a and compare each of them to the corresponding element in b, thus making sure that each element has exactly the same value and the same position in both arrays.

This worked, but I was still very confused as to why a === b would return false even when a and b were identical arrays, so I checked MDN Web Docs and found this:

The identity operator returns true if the operands are strictly equal (see above) with no type conversion.

More confusion followed, until I found this:

Note that the distinction between these all have to do with their handling of primitives; none of them compares whether the parameters are conceptually similar in structure. For any non-primitive objects x and y which have the same structure but are distinct objects themselves, all of the above forms will evaluate to false.

Basically, primitive types like strings and numbers are compared by their value, while objects like arrays, dates, and plain objects are compared by their reference. In other words, our two game states are arrays that point to different locations in memory, so the comparison currentGameState === nextGameState is never true.

I still haven’t figured out why but at least I understand what it’s doing and it was fun to investigate!

I come across stuff like this in Javascript all the time so I’m sure there’ll be more posts like this to come.