An example race condition in JavaScript

race-condition.js

// An example race condition in JavaScript
// When you run this script using Node or in a browser, you'll find it
// does not print "Ended with 0", but a random number. Even though the functions running
// simply loop 100 iterations of adding and subtracting. The reason the end result is random
// is because the sleeps are of random duration and the time between the read of the variable
// causes the eventual write to be incorrect when `adder` and `subber` interleave.
// This problem is similar to https://en.wikipedia.org/wiki/Time-of-check_to_time-of-use

let number = 0;
const times = 100;

function sleep() {
  return new Promise((resolve) => setTimeout(resolve, Math.random() * 5));
}

async function adder() {
  for (let i = 0; i < times; i++) {
    await sleep();
    let read = number;
    read = read + 1;
    await sleep();
    number = read;
  }
}

async function subber() {
  for (let i = 0; i < times; i++) {
    await sleep();
    let read = number;
    read = read - 1;
    await sleep();
    number = read;
  }
}

async function main() {
  console.log("Started with", number);
  await Promise.all([
    adder(),
    subber(),
  ]);
  console.log("Ended with", number);
}

main()
  .then(() => console.log("All done"))
  .catch((err) => console.error(err));