bench-beyond-length.js

// Benchmark to measure loops running until OOB vs. loops that run
// to length. See
//
//  https://v8project.blogspot.de/2017/09/elements-kinds-in-v8.html#avoid-reading-beyond-length
//
// for background. While OOB access is now no longer almost 10x
// slower in V8, it's still an anti-pattern that should be avoided.

"use strict";

function doSomething(item) {}

function testArrayInBounds(items) {
  for (let i = 0; i < items.length; ++i) {
    doSomething(items[i]);
  }
}

function testArrayOutOfBounds(items) {
  for (let i = 0, item; (item = items[i]) != null; ++i) {
    doSomething(item);
  }
}

function testStringInBounds(string) {
  for (let i = 0; i < string.length; ++i) {
    doSomething(string[i]);
  }
}

function testStringOutOfBounds(string) {
  for (let i = 0, char; (char = string[i]) != null; ++i) {
    doSomething(char);
  }
}

const N = 1e7;
const F = [
  testArrayInBounds,
  testArrayOutOfBounds,
  testStringInBounds,
  testStringOutOfBounds
];
const items = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

function test(f, n) {
  var result;
  for (var i = 0; i < n; ++i) {
    result = f(items);
  }
  return result;
}

// Warmup
test(x => x, 50);
for (var i = 0; i < 10; ++i) {
  for (var f of F) test(f, 50);
}

// Measure
for (var f of F) {
  var start = Date.now();
  var o = test(f, N);
  var end = Date.now();
  console.log(f.name + ": " + (end - start) + " ms.");
}