A speed test between using concat() and push() when reducing an array of arrays to a single array

array-concat-push-speed-test.js

// Which is faster depends on which version of Node you're using.
// See the bottom for the results.

const iterations = 1000000;
const arrayOfTen = [...Array(10).keys()];
const arrayOfArrays = [...Array(10).keys()].map(() => arrayOfTen);
const NS_PER_SEC = 1e9;

function a() {
  for (let i = 1; i < iterations; i++) {
    arrayOfArrays.reduce((acc, innerArray) => {
      acc.push(...innerArray);
      return acc;
    }, []);
  }
}

function b() {
  for (let i = 1; i < iterations; i++) {
    arrayOfArrays.reduce((acc, innerArray) => acc.concat(innerArray), []);
  }
}

let time = process.hrtime();
a();
let diff = process.hrtime(time);
console.log(`Test A took ${diff[0] * NS_PER_SEC + diff[1]} nanoseconds`);

time = process.hrtime();
b();
diff = process.hrtime(time);
console.log(`Test B took ${diff[0] * NS_PER_SEC + diff[1]} nanoseconds`);

// Results:

// $ node - v
// v8.3.0

// Test A took 653492137 nanoseconds
// Test B took 1968402981 nanoseconds

// Test A took 649657112 nanoseconds
// Test B took 1961882017 nanoseconds

// Test A took 635760306 nanoseconds
// Test B took 1862738481 nanoseconds

// $ node - v
// v6.11.2

// Test A took 10184520458 nanoseconds
// Test B took 3388628276 nanoseconds

// Test A took 9476108180 nanoseconds
// Test B took 3162485775 nanoseconds

// Test A took 9456780696 nanoseconds
// Test B took 3167964775 nanoseconds