resolve values in a Promise While Loop

Question

I found: Correct way to write loops for promise. and While loop using bluebird promises

However when I try to use these to loop a promise, the resolve value of that promise is not passed down the chain.

For example, the following code prints:

did it 1 times
did it 2 times
undefined <-- this should be hello world

var Promise = require('bluebird');

var times = 0;

var do_it = function(o) {
    return new Promise(function(resolve, reject) {
        setTimeout(function () {
            console.log("did it %d times", ++times);
            resolve(o);
        }, 1000);
    });
}

var promiseWhile = Promise.method(function(condition, action) {
    if (!condition()) return;
    return action().then(promiseWhile.bind(null, condition, action));
});

var do_it_twice = function(o) {
    return promiseWhile(
        function() { return times < 2; },
        function() { return do_it(o);  }
    );
}

do_it_twice("hello world").then(function(o) {console.log(o)});

`if (!condition()) return;` - change that to `if (!condition()) return 'oops';` ... what happens now? — Jaromanda X, Feb 08 '16 at 11:33
really? that doesn't solve the problem, that shows you what you are doing wrong! — Jaromanda X, Feb 08 '16 at 11:40

score 3 · Accepted Answer · answered Feb 08 '16 at 11:40

You need to specify returned value

var promiseWhile = Promise.method(function(condition, action, result) {
    if (!condition()) return result;
    return action().then(promiseWhile.bind(null, condition, action, result));
});

var do_it_twice = function(o) {
    return promiseWhile(
        function() { return times < 2; },
        function() { return do_it(o);  },
        o
    );
}

score 1 · Answer 2 · answered May 29 '18 at 16:27

I recently had a problem similar to this problem and came across this question. I ended up using an asynchronous recursive function to loop the request until a condition was met. In my case I had to check the length of an array attribute of the response (it was scraped data and a script on the page would sometimes not load the data before sending a response). Here was my solution, which sends another request if the response contained no articles (it is vanilla ES6 and allows you to use any condition (I used out.data.length===0)):

export async function scrape () {
    let out =  await axios.get('/api/scrape');
    if(out.data.articles.length===0){
        return await scrape();
    }
    else return out;
}

resolve values in a Promise While Loop

2 Answers2