lock.js

// idea, we want to start two async processes a and b.

const a = async () => { do_async_stuff() }
const b = async () => { do_async_stuff() }

// but we need to isolate them. So b should not run before a

const result = Promise.all([a, b]) 

// this way they can overlap, not good. So lets use a lock

const lock = createLock()
const a = async () => lock.atomic(() => do_async_stuff())
const b = async () => lock.atomic(() => do_async_stuff())

// this implements createLock:

function createLock() {
  let locked = false
  const atomic = (fun) => {
    return new Promise(function(resolve, reject) {
      const rec = () => {
        if (locked) {
          setTimeout(rec, 0)
        } else {
          locked = true
          fun().then(res => {
            resolve(res)
            locked = false
          }, (err) => {
            reject(err)
            locked = false
          })
        }    
      }
      rec()
    })
  }

  return {
    atomic
  }
}