arrayFlatBenchmark.js

// https://stackoverflow.com/questions/61411776/is-js-native-array-flat-slow-for-depth-1
// [--jsperf is down--] https://jsperf.com/flat-array-depth-1 - I added this jsperf benchmark after this gist was created and the SO question was asked.

let TenThouWideArray = Array(1000).fill().map(el => Array(10).fill(1));
let TenThouNarrowArray = Array(10).fill().map(el => Array(1000).fill(1));
let TenMilWideArray = Array(10000).fill().map(el => Array(1000).fill(1));
let TenMilNarrowArray = Array(100).fill().map(el => Array(100000).fill(1));

let benchmarks = { TenThouWideArray, TenThouNarrowArray, TenMilWideArray, TenMilNarrowArray };
let implementations = [
    flattenPreAllocated,
    flattenNotPreAllocated,
    function nativeFlat(arr) { return Array.prototype.flat.call(arr) },
    function spreadThenConcat(arr) { return [].concat(...arr) }
];

let result;
Object.keys(benchmarks).forEach(arrayName => {
    console.log(`\n............${arrayName}............\n`)
    implementations.forEach(impl => {
        console.time(impl.name);
        result = impl(benchmarks[arrayName]);
        console.timeEnd(impl.name);
    })
    console.log(`\n............${arrayName}............\n`)
})


function flattenPreAllocated(arr) {
    let numElementsUptoIndex = Array(arr.length);
    numElementsUptoIndex[0] = 0;
    for (let i = 1; i < arr.length; i++)
        numElementsUptoIndex[i] = numElementsUptoIndex[i - 1] + arr[i - 1].length;
    let flattened = new Array(numElementsUptoIndex[arr.length - 1] + arr[arr.length - 1].length);
    let skip;
    for (let i = 0; i < arr.length; i++) {
        skip = numElementsUptoIndex[i];
        for (let j = 0; j < arr[i].length; j++)
            flattened[skip + j] = arr[i][j];
    }
    return flattened;
}
function flattenNotPreAllocated(arr) {
    let flattened = [];
    for (let i = 0; i < arr.length; i++) {
        for (let j = 0; j < arr[i].length; j++) {
            flattened.push(arr[i][j])
        }
    }
    return flattened;
}