Benchmarks demonstrating interesting properties of optimization around function arity in JavaScript

functionArityOptimization.ts

import Benchmark from "benchmark";

const array: number[] = [];
for (let i = 0; i < 100000; i++) {
    array.push(i);
}

function map<T, U>(arr: T[], f: (item: T) => U): U[] {
    const result: U[] = [];
    for (let i = 0, { length } = arr; i < length; i++) {
        result.push(f(arr[i]));
    }
    return result;
}

function mapIndexed<T, U>(arr: T[], f: (item: T, index: number) => U): U[] {
    const result: U[] = [];
    for (let i = 0, { length } = arr; i < length; i++) {
        result.push(f(arr[i], i));
    }
    return result;
}

function mapOptimized<T, U>(arr: T[], f: (item: T, index: number) => U): U[] {
    const needsIndex = f.length > 1;
    const result: U[] = [];
    for (let i = 0, { length } = arr; i < length; i++) {
        result.push(needsIndex ? f(arr[i], i) : (f as any)(arr[i]));
    }
    return result;
}

new Benchmark.Suite()
    .add("map()", () => {
        map(array, x => 2 * x);
    })
    .add("mapIndexed() with one arg", () => {
        mapIndexed(array, x => 2 * x);
    })
    .add("mapIndexed() with two args", () => {
        mapIndexed(array, (x, _) => 2 * x);
    })
    .add("mapOptimized() with one arg", () => {
        mapOptimized(array, x => 2 * x);
    })
    .add("mapOptimized() with two args", () => {
        mapIndexed(array, (x, _) => 2 * x);
    })
    .on("cycle", function(event: any) {
        console.log(String(event.target));
    })
    .run({ async: true });

// map() x 523 ops/sec ±1.10% (86 runs sampled)
// mapIndexed() with one arg x 412 ops/sec ±1.75% (82 runs sampled)
// mapIndexed() with two args x 508 ops/sec ±2.82% (87 runs sampled)
// mapOptimized() with one arg x 512 ops/sec ±2.68% (87 runs sampled)
// mapOptimized() with two args x 514 ops/sec ±1.68% (86 runs sampled)