JavaScript | 4 mins read

August 4, 2023

Array concat in Javascript - a performance perspective

Sometimes, what we think is slower might be faster.

I recently encountered a problem where I had to optimize a function that handles an array size of more than 10k items. The goal of the function was just to determine if the items in the multidimensional array are all empty.

Here’s what the array looks like:

    { some: "", unique: "", key: "", value: "", pair: "" }

The original function had a logic similar to this:

let allValues = [];
outerArray.forEach((innerArray) => {
  innerArray.forEach((obj) => {
    allValues = allValues.concat(Object.values(obj));

// If we found one, then there are results
const hasNoResults = !allValues.find(Boolean);

The logic is pretty easy to understand. However, it isn’t efficient, and it’s not because of the nested array.

We encountered a couple of cases where we had to iterate over 10k items in the first level of the multidimensional array, and this caused the UI to hang for a couple of seconds, with the maximum we saw around 30 seconds in Chrome.

Who’s the culprit?

We used Chrome’s DevTools’ Performance profiler to dig into which exact line was taking up the most time.

In the screenshot above, you can also notice the yellow area chart at the very top. That’s the CPU usage, which you can see is hovering at 100% for 33 seconds.

The specific line that is the culprit is the line where we did the .concat.

The solution

There were two solutions that were floated:

  • One was to use push and mutate the array, retaining most of the current logic in place.
  • The other was to add another nested loop, processing each item rather than building a new array with the combined values before doing the checking.

We went for the latter solution as after some benchmarking, the worst-case scenario has the same benchmark for the two options, but the best-case scenario is faster when we simply short-circuit the loops without having to copy items to a temporary array.

Here’s a rough code of the benchmark I made on my work laptop:

// test data
const data = Array.from(new Array(10000)).map((i) => ({
  v: 'rand_' + i,
  z: 'rrrr' + i,
  l: i % 2 ? '0' : '0'

function arrayConcat() {
  let values = [];
  data.forEach((d) => {
    values = values.concat(Object.values(d));
  const hasNoResults = values.every((a) => a === "0");
  console.log('array.concat noResults', hasNoResults);

function nestedLoops() {
  console.time('nested loops')
  const hasNoResults = typeof data.find((d) => {
    return typeof Object.values(d).find((v) => v !== '0') !== 'undefined'
  }) === 'undefined';
  console.log('nested loops noResults', hasNoResults);
  console.timeEnd('nested loops');

function arrayPush() {
  let values = [];
  data.forEach((d) => {
  const hasNoResults = values.every((a) => a === "0");
  console.log('array.push noResults', hasNoResults);


The results of the benchmark are:

array.concat: 58.93701171875 ms
nested loops: 0.028076171875 ms
array.push: 0.910888671875 ms

Even on the worst case scenario, nested loops logged in very similar time as Array.push, so we decided to go for nested loops instead of the arguably a lot readable Array.push approach.

The possible reason

Although we never dug into how V8 behaves with regard to Array concat and push, we were able to find a couple of articles to back up our decision of ditching concat.

We found this comment in a article that sort of explains the possible reason:

Using array concat copies values to a new array.

When done repeatedly, it also means increased memory access.

When dealing with large arrays, where the data may exceed the bounds of the L0 cache of the CPU, then the CPU would most likely need to access a much more distant cache.

This could explain the noticeable increase in CPU usage while the browser is hanging.


Does this mean that we should always avoid using concat? No, not really. We should just be aware of when not to use it.

It was 2015 when I first caught wind of React and its push with immutability, and the succeeding years were filled with people evangelizing why we should not mutate objects and arrays.

The reasons are all valid, but as with almost everything in life, we should always have a balanced view of things and understand when to use an approach, and when not to. This is a great example for us to be careful about treating everything we learn as absolutes.

Always, always evaluate the scenario when choosing what solution to go for.


Javascript Array.push is 945x faster than Array.concat

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