Control while loop by Promise then function

Question

I have a function that returns Promise

let processWork = function () {
  return new Promise(resolve => {
    console.log('- internal: start')
    // just wait 200ms for example
    let future = new Date(new Date().getTime() + 200)
    while (future > new Date()) {}
    console.log('- internal: done')
    resolve()
  })
}

I call it and use then function to change variable to break endless while but not work

let doing = false
let done = false
while (!done) {
  if (doing) {
    continue
  }

  console.log('Starting work')
  doing = true
  processWork()
    .then(() => {
      console.log('Finished')
      done = true
    })
}

I get output

Starting work
- internal: start
- internal: done

and my code's still running forever. My question is why then function not running in this case.

Answer 1

It is not true that a promise will somehow get resolved while a busy loop is being executed. As long as a loop is executing no other JavaScript will execute (excluding web workers). The following loop will always execute the continue statement in every iteration and will never finish.

let doing = false
let done = false
while (!done) {
  if (doing) {
    continue
  }
  // ...
}

No promise could help interrupt this loop, but in your case, there is not even a promise since processWork never gets executed.

If processWork would get executed, the loop in the promise constructor callback will lock up the browser until the target time arrives: no other JavaScript gets executed during that delay. This defeats the purpose of promises: they are intended not to lock up the browser and allow the execution of other code while the promise is still pending.

So, do it like this:

 let processWork = function () { return new Promise(resolve => { console.log('- internal: start delay of 200ms') // just wait 200ms for example, but don't stop execution of other code setTimeout(resolve, 200); }) } console.log('Starting work') processWork().then(() => { console.log('Promise resolved. Finished'); }); console.log('Waiting for promise to resolve...'); console.log('In the meanwhile let\\'s calculate 10!'); let fact = 1; for (let i = 1; i <= 10; i++) { fact *= i; } console.log('10! = ', fact);