using callback in a function in node.js

Question

I had a code like this :

router.post('/call' , async (req , res)=>{
    try{
        await async.mapLimit(req.body.n , 5 , (id , callback)=> {
            ////do something
            callback("message")
        }, (err , rest)=>{
            if(err){
                res.json({message : err})
            }else{
                res.json({message : rest})
            }
        })

    }catch(err){
        res.json({message : err})
    }
})

and i want to make the "do something part" a function like this:

router.post('/call' , async (req , res)=>{
    try{
        await async.mapLimit(req.body.n , 5 , (id , callback)=> {
            addStudentID(req , res , id , callback)
        }, (err , rest)=>{
            if(err){
                res.json({message : err})
            }else{
                res.json({message : rest})
            }
        })

    }catch(err){
        res.json({message : err})
    }
})

my problem is that it seems that callback can not sent as a parameter to another function is there any solution to do that?

`await` only does something useful when you `await` a promise. So, awaiting a function that takes a callback and does not return a promise will NOT wait for the callback to be called. — jfriend00, Sep 15 '19 at 06:46
Make sure that all of your functions use async/await or promises, and use a for loop instead of the async module — Nicolas El Khoury, Sep 15 '19 at 07:35

nnfans · Answer 1 · 2019-09-15T10:38:37.980

First, async in function declaration is useless unless you await promise function (async-await function). But I don't know why you passing a function as a callback to a promise function (async-await function).

Then, let's assume that your async.mapLimit is callback based function. You can wrap your callback function with promise then produce async-await function (function that return promise) like below:

function async.mapLimitPromise(firstArgument, secondArgument) {
  return new Promise(function(resolve, reject) {
    async.mapLimit(firstArgument, secondArgument, function(err, data) {
    if (err) {
      reject(err)
    }
    resolve(data)
  })
})

But one more thing. I don't understand why you pass a function (your callback) as a 3rd argument of a async.mapLimit function, but then it receive a callback as second argument

I recommend you to read more about callback function first, then a promise, and last you can try async function

---Edit---

Try with this code first:

router.post('/call' , async (req , res)=>{
  try{
    const res = await async.mapLimit(req.body.n , 5 , (id , callback)=> {
      addStudentID(req , res , id , callback)
    })
    res.json({message : rest})

  }catch(err){
    res.json({message : err})
  }
})

async call was because of an await function in the do something but maybe you are right it is useless in maplimit function but callback argument was ok — behnam, Sep 15 '19 at 08:33

score 0 · Accepted Answer · answered Sep 15 '19 at 08:35

0

problem was problem with function usage and function declaration that were not sync not the callback part

answered Sep 15 '19 at 08:35

behnam

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using callback in a function in node.js

2 Answers2