Javascript async pool - exec async functions in a pool of fixed sized

asyncPool.js

/** Async pool
 * Originally seen at https://github.com/rxaviers/async-pool/blob/master/lib/es6.js
 * Simplified thanks to u/GSLint in
 * https://www.reddit.com/r/learnjavascript/comments/gebobv/cant_grok_asyncpool_es6_code/
 */
 
 /** Run asyncFunction array, poolSize at a time. */
async function asyncPool (array, poolSize) {
  const result = []
  const pool = []

  // Promises leave the pool when they're resolved.
  function leavePool (e) { pool.splice(pool.indexOf(e), 1) }

  for (const item of array) {
    const p = Promise.resolve(item())
    result.push(p)
    const e = p.then(() => leavePool(e))
    pool.push(e)
    if (pool.length >= poolSize)
      await Promise.race(pool)
  }
  return Promise.all(result)
}

/** Sample async workers. */
const resolveInXSeconds = (x) => {
  return new Promise((resolve) => {
    console.log('starting ' + x)
    setTimeout(() => resolve(x), x * 1000)
  })
}
const r2 = () => resolveInXSeconds(2)
const r3 = () => resolveInXSeconds(3)
const r5 = () => resolveInXSeconds(5)

asyncPool( [r2, r3, r5, r2, r3, r2, r5, r2, r2], 5 ).
  then(results => console.log(results.join()))