How to pause / resume / delay a for loop? [duplicate]

Question

How would you pause / resume / delay a for loop in Javascript ?

Let's say I have something like this

for (let i = 0; i < 10; i++) {
   console.log('value of i is',i)
}

We can delay an entire function using something like this, but it does not work with a for loop

function delayAFn(ms){
    return new Promise(resolve => setTimeout(() => {
        resolve();
    }, ms))
}

You would be better off not using a loop, just use setTimeout or setInterval and keep track. — epascarello
– epascarello, Commented Sep 6, 2019 at 3:47
Could you give more context as to why you need the delay? For example, if you want the for-loop to go slower, add a nested for-loop inside. But if you need to do some async code, then you may need a Promise. — ctaleck
– ctaleck, Commented Sep 6, 2019 at 4:07
@ChristopherTaleck , I have a data graph, I have the data in an array. But don't want to map the data immediately, the user will be given the option of the speed the data need to be mapped. The user may also pause the execution. Similar to what we do using the browser debugger. — Gaurav_soni
– Gaurav_soni, Commented Sep 6, 2019 at 4:19
Not a duplicate question. This question specifically asked how to pause the delay also. — ctaleck
– ctaleck, Commented Sep 6, 2019 at 4:56

epascarello · Accepted Answer · 2019-09-06 04:01:41Z

1

What we would have done in the past is a queue type of set up where you use a timeout to call the next iteration if it is still good.

function nextStep(i) {
  console.log(`value of i is ${i}`)
  if (i < 9) {
    window.setTimeout(nextStep, 1000, ++i)
  } else {
    console.log('done')
  }
}
nextStep(0)

answered Sep 6, 2019 at 4:01

epascarello

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Comments

William Chong · Accepted Answer · 2019-09-06 03:44:30Z

0

You can use async/await syntax inside a loop.

for (let i = 0; i < 10; i++) {
   console.log('value of i is',i)
   await delayAFn(1000);
}

answered Sep 6, 2019 at 3:44

William Chong

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2 Comments

Gaurav_soni Over a year ago

Ok , and is it possible to pause the loop? So let's say the loop is running at 2 sec time interval. I pause / resume it using some button.

William Chong Over a year ago

replace delayAFn with a promise that resolves only when a button is pressed

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