Wait for async.maplimit using promise

Question

I am attempting to use promise to wait for async.mapLimit. I use it to run several shell scripts at the same time and I want to wait for all of them to finish executing before I continue past the log 'the end'. But I always get a undefined when using the return value for promise.

var myArray = [5,1,2,3,4];

const async = require('async');
const exec = require('child_process').exec;

function understandPromise() {

    const _waitForMe = async.mapLimit(myArray, 16, doSomeThing, function(err, results){
        console.log(results.length, 'should equal (doSomeThing)', myArray.length);
        console.log('err',err);

    });

    // This also gives undefined
    //const _waitForMe = async.mapLimit(myArray, 16, doSomeThing).then(a,b);
    _waitForMe.then(a,b);

    console.log('the end');

}

function doSomeThing(item, callback){

    let runCmd = './test.sh ' + item;
    console.log('before', runCmd);
    exec(runCmd, function (error, stdout, stderr) {
        console.log('error', error);
        console.log('stderr', stderr);
        console.log('stdout', stdout);
        console.log('after', runCmd);
        callback(null, item); // or actually do something to item
    });

}

understandPromise();

TypeError: Cannot read property 'then' of undefined relating to _waitForMe

Why doesn't mapLimit return a promise? I realize I am doing something fundamentally wrong here but i cannot figure out what. Other solutions not involving promise is also something I could consider.

Skipping the callback yields the same problem 'then' of undefined

const _waitForMe = async.mapLimit(myArray, 16, doSomeThing);

Similar SO questions like this only gives errors async.mapLimit with Promise

Update 1 after comments Commenters have suggested this:

async function understandPromise() {
        let results = await async.mapLimit(myArray, 8, doSomeThing);
        console.log('results', results.length);
        console.log('the end');
}

But this leads to (node:567) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'length' of undefined.

I still can't understand why the return of async.mapLimit is undefined when there is no callback involved.

Update 2

I got this to work from the comments but in the end it was that the async module was at an old version 1.5.2 and not at the newer 3.2.0

The version you put in comments, works [fine](https://repl.it/@trincottrincots/UsingAsyncMapLimit) (of course, none of the shell scripts will succeed in this demo, but that is not relevant). Check version of `async` package is the latest? — trincot, Feb 27 '20 at 15:57
@trincot I tried your code, but it still runs past console.log('the end') before all of the shell script/your sleep has finished. ( async@3.2.0 ) — bits, Feb 27 '20 at 23:55
Of course, you execute that synchronously. If you don't want that, then you must put that in function 'b'. — trincot, Feb 28 '20 at 06:59

Ashish Modi · Answer 1 · 2020-02-27T15:41:54.827

0

The problem is you are passing the callback at the end of the function. The documentation clearly says it returns a promise, if no callback is passed. Remove function(err, results) and it should start returning a promise.

const results = await async.mapLimit(myArray, 16, doSomeThing);
console.log(results.length, 'should equal (doSomeThing)', myArray.length);

edited Feb 27 '20 at 15:41

answered Feb 27 '20 at 15:18

Ashish Modi

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Do you mean like this? var _waitForMe = async.mapLimit(myArray, 16, doSomeThing); _waitForMe.then(a,b); This also becomes undefined – bits Feb 27 '20 at 15:40
@bits sorry what do you mean? I have added the code as well. – Ashish Modi Feb 27 '20 at 15:42
const results = await async.mapLimit(myArray, 16, doSomeThing); SyntaxError: await is only valid in async function, so this cannot work – bits Feb 27 '20 at 15:48
1

@bits, Of course you need to put that in an `async` function; that goes without saying. You can execute that async function immediately like an IIFE. – trincot Feb 27 '20 at 16:00
`async function understandPromise() {` – Ashish Modi Feb 27 '20 at 16:13

Wait for async.maplimit using promise

1 Answers1