Flattening an Array, a Performance Test

The Question

What is the most efficient way to flatten an array in Javascript? This questions comes up often in interviews and also has some practical use cases, which is rare for interview questions.

By "flattening" an array, we mean taking a multidimensional array and turning it into a regular "single" dimensional array. I'm not sure if that is the right phrasing but I'm going with it. See an example below.

let initialArray = [[0, 1], [2, 3], [4, 5]];

let flattenedArray = [0, 1, 2, 3, 4, 5]

Possible Solutions

So what is the most efficient way to go about this? Well I sketched out four possible solutions.

Solution 1: Concat + Apply

let flattenedArray = Array.prototype.concat.apply([], initialArray);

This is a fairly simple one line solution. We create a new array and concat each element onto it.

Solution 2: Use a Reduce

let flattenedArray = initialArray.reduce((a, b) => {
  return a.concat(b);
});

We execute a concat function on each first level element in the array. Then concat it onto the previous element. Giving us a new flattened array.

Solution 3: The Faithful Loop

let flattenedArray = [];
for (let i = 0; i < initialArray.length; i++) {
  let current = initialArray[i];
  for (let j = 0; j < initialArray.length - 1; j++)
    flattenedArray.push(current[j]);
}

The most basic solution. We loop through the first level arrays, loop through the internal arrays and push them up to an empty array. I should note, this is a lot more written code than our other solutions.

Solution 4: A ForEach Loop

let flattenedArray = [];
initialArray.forEach(entry => {
  flattenedArray = flattenedArray.concat(entry);
});

A little more modern approach. We loop through each first level array, concat it with the other and reassign the flattenedArray.

Testing the Results

So which is the fastest?

Full test results here . Wow! The old fashioned "for" loop was by far the fastest.

Going be a straight operations per second metric the classical for loop performs best! This was actually fairly shocking to me. Does anyone have a better flattening method?