Run results for: Flatten Arrays - Large

Run details:

User agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/85.0.4183.102 Safari/537.36

Browser: Chrome 85

Operating system: Windows

Device Platform: Desktop

Date tested: 4 years ago

Test name	Executions per second
concat	315107.0 Ops/sec
spread	45599.4 Ops/sec
reduce spread	42622.2 Ops/sec
reduce concat	325191.4 Ops/sec
flat	34693.8 Ops/sec
concat apply	991974.8 Ops/sec
Flatten self-made	37145.7 Ops/sec

Script Preparation code:

const size = 100000
const innerSize = size
function getInnerArray() {
  return new Array(size).fill().map(() => parseInt(Math.random() * innerSize + 1))
}
var BaseArrays = new Array(size).fill().map(() => getInnerArray())
function concatArray(finalArray) {
   BaseArrays.forEach((array) => {
     finalArray = finalArray.concat(array)
   })
}
function spreadArray(finalArray) {
   BaseArrays.forEach((array) => {
     finalArray = [...finalArray, ...array]
   })
}
function reduceArraySpread(finalArray) {
   finalArray = BaseArrays.reduce((prev, current) => [...prev, ...current], [])
}
function reduceArrayConcat(finalArray) {
   finalArray = BaseArrays.reduce((prev, current) => prev.concat(current), [])
}
function flatArray(finalArray) {
   finalArray = BaseArrays.flat()
}
function mergedArray(finalArray) {
   finalArray = [].concat.apply([], BaseArrays);
}
function flatten (arr, finalArray = []) {
  for (let i = 0, length = arr.length; i < length; i++) {
    const value = arr[i];
    if (Array.isArray(value)) {
      flatten(value, finalArray);
    } else {
      finalArray.push(value);
    }
  }
  return finalArray;
};

Tests:

concat
let finalArray = [] concatArray(finalArray)
spread
let finalArray = [] spreadArray(finalArray)
reduce spread
let finalArray = [] reduceArraySpread(finalArray)
reduce concat
let finalArray = [] reduceArrayConcat(finalArray)
flat
let finalArray = [] flatArray(finalArray)
concat apply
let finalArray = [] mergedArray(finalArray)
Flatten self-made
let finalArray = [] flatten(BaseArrays, finalArray)

Run results for: Flatten Arrays - Large

What is the best way to flatten 2d arrays into 1 array of items

concat

spread

reduce spread

reduce concat

flat

concat apply

Flatten self-made