Is the map method parallel?

Question

Is the method [1, 2, 3].map(item=>f(item)) execute f(1), f(2), f(3) parallelly? Or execute like "f(1), then f(2), then f(3)"?

Answer 1

Actually it execute "f(1), then f(2), then f(3)"?

Because JS is single-threaded. So if the f function is an async task (setTimeout, Ajax call, etc..), then it can be scheduled to waits parallelly combined with promise.all. Otherwise, it still has to execute synchronously.

---

One more thing to keep in mind is when you use await inside async function, It does only waits parallel when you use promise.all.

var tick;
const log = (v) => console.log(`${v} \n Elapsed: ${Date.now() - tick}`);

const f = (number) => new Promise(resolve => {
      setTimeout(() => log(number), 1000);
   });

async function Run(){
  tick = Date.now();
  await Promise.all([f(1), f(2), f(3)]);
}

function Run_2(){
  tick = Date.now();
  [f(1), f(2), f(3)];
}

Run();
Run_2();

While you await each f, it takes 3s in total.

var tick;
const log = (v) => console.log(`${v} \n Elapsed: ${Date.now() - tick}`);

const f = (number) => new Promise(resolve => {
      setTimeout(() => resolve(number), 1000);
   });

async function Run(){
  tick = Date.now();
  let a = await f(1);
  let b = await f(2);
  let c = await f(3);
  
  log([a, b, c]);
}
Run();

However, Promise.all can only waits async tasks in parallel. It can't convert a synchronous CPU block code into a separate thread or anything.