Create infinite loop with Promise recursion in NodeJS

Question

I'm getting rid of my blocking infinite while loop and substituting it with promises. I have a simple run function which lights up an LED and turns it off after which it continues to the next one.

Obviously Promises don't work inside while loops so I was wondering how I can use recursion to go back into my run() function and also increment my counter x?

var x = 0
var sign = 1
function run() {
    return new Promise((resolve, reject) => {
        try {
            wait(20).then(() => { device.setColor("random", { 'channel': 0, 'index': x }) })
            wait(20).then(() => { device.setColor("#000000", { 'channel': 0, 'index': x }) })
            x += sign
            if (x == LED_COUNT - 1) {
                sign = -1
            } else if (x == 0) {
                sign = 1
            }
            return resolve()
        } catch (e) {
            return reject(e)
        }
    })
}
run() // this only runs once, I need it to run forever

You may find this answer helpful: https://stackoverflow.com/a/24660323/253298 — duckbrain, Oct 25 '17 at 19:02

Dalin Huang · Accepted Answer · 2017-10-25T19:28:51.233

2

Recursive call:

add .then(run()) to the end of your promise.

not sure how your wait works, I'm using a setTimeout to delay each loop by 1 sec. the example below:

var x = 0
var sign = 1
function run() {
    return new Promise((resolve, reject) => {
        try {
            //example only
            console.log('x-->' + x);
            //wait(20).then(() => { device.setColor("random", { 'channel': 0, 'index': x }) })
            //wait(20).then(() => { device.setColor("#000000", { 'channel': 0, 'index': x }) })
            x += sign
            if (x == LED_COUNT - 1) {
                sign = -1
            } else if (x == 0) {
                sign = 1
            }
            return resolve()
        } catch (e) {
            return reject(e)
        }
    }).then(setTimeout(function(){ run() }, 1000));
}
run();

edited Oct 25 '17 at 19:28

answered Oct 25 '17 at 19:04

Dalin Huang

11,212
5
32
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this causes the max call stack to be exceeded – Nick Oct 25 '17 at 19:28
@Ortix92 yes, I'm adding a delay of 1 sec. the op is asking for an infinite loop, which doesn't exist, but can be done with the recursive call. – Dalin Huang Oct 25 '17 at 19:29
My `wait()` works with a promise as well! `const wait = ms => new Promise(resolve => setTimeout(resolve, ms)); ` – Nick Oct 25 '17 at 19:32

Amit Wagner · Answer 2 · 2020-07-04T05:17:18.837

1

not sure why you need a promise here.

var x = 0
var sign = 1

function run() {

    setTimeout(function () {
        device.setColor("random", {'channel': 0, 'index': x})
        device.setColor("#000000", {'channel': 0, 'index': x})
        if (x == LED_COUNT - 1) {
            sign = -1
        } else if (x == 0) {
            sign = 1
        }

        run()
    }, 20000)


}
run()

edited Jul 04 '20 at 05:17

answered Oct 25 '17 at 19:10

Amit Wagner

3,134
3
19
35

1

Thank you for making so much sense here! – Justin Hamilton Jul 01 '20 at 17:08

MinusFour · Answer 3 · 2017-10-25T19:18:16.943

I'd assume you'd want to wait for both promises and then run it again right? Here's a way you could try it. If you have access to Promise.try use that instead of Promise.resolve().then(...

var x = 0
var sign = 1

function run() {
  return Promise.resolve().then(function() {
    x += sign
    if (x == LED_COUNT - 1) {
      sign = -1
    } else if (x == 0) {
      sign = 1
    }
    return Promise.all([
      wait(20).then(() => {
        device.setColor("random", {
          'channel': 0,
          'index': x
        })
      }),
      wait(20).then(() => {
        device.setColor("#000000", {
          'channel': 0,
          'index': x
        })
      })
    ]);
  }).then(run);
}
run()

You could also just group both of them in one function and only use one wait(20) promise:

return wait(20).then(() => {
    device.setColor("random", {
        'channel': 0,
        'index': x
    });
    device.setColor("#000000", {
        'channel': 0,
        'index': x
    });
})

Create infinite loop with Promise recursion in NodeJS

3 Answers3