async.mapLimit does not return a Promise

What version of async are you using? 3.1.0

Which environment did the issue occur in (Node version/browser version) node 12.9.1, npm 6.10.2, browser N/A

What did you do? Please include a minimal reproducable case illustrating issue. Issue has a thread in stackoverflow https://stackoverflow.com/questions/57622495/async-maplimit-with-promise/57659221#57659221

Basically, I have this code:

async = require('async');
let numPromise = async.mapLimit(['1','2','3','4','5'], 3, function(num, callback){
    setTimeout(function(){
        num = num * 2,
        console.log(num);
        callback(null, num);
    }, 
    2000);
})
numPromise
.then((result) => console.log("success:" + result))
.catch(() => console.log("no success"));

What did you expect to happen? Execute without errors, ‘numPromise’ should contain a Promise. console should log ‘2,4,6,8,10’ and ‘success:2,4,6,8,10’

What was the actual result? It throws an error: TypeError: Cannot read property ‘then’ of undefined

Note: When I use the ‘promise-async’ module instead of ‘async’ then this code works well. Documentation says that async.mapLimit (and others) return a Promise when no callback is supplied, but I get undefined. Couldn’t find any working sample yet (please also see my suggestion on the ‘need samples’ issue).

[caub]

const async = require('async');
const delay = require('util').promisify(setTimeout);
const numPromise = async.mapLimit(['1','2','3','4','5'], 3, async num => delay(200).then(() => num*2))
// or const numPromise = async.mapLimit(['1','2','3','4','5'], 3, async num => {
//    await delay(200);
//    return num*2;
// })
numPromise.then(console.log)

[aearly]

Wrap the async function in asyncify. http://caolan.github.io/async/v3/global.html#AsyncFunction