Loop with Asynchronous Calls

Question

I could not get the index right of for loop when I asynchronous function inside a for loop. Here is my code:

for (var i = 0; i < details.waypoints.meta.length; i++) {
    for (var j = start_index; j < i; j++) {
        (function (cntr) {
            fs.readFile(Path.join(__dirname, "/../../Modified_Data.json"), "utf8", function readFileCallback(err, data) {
                if (err) {
                    console.log(err);
                    return;
                } else {
                    console.log(cntr) //printing the index
                }
            });
        })(j);
    }
}

The index I am getting are which are not in right order: 1 3 0 2 4 5 6 7 8 9

Possible duplicate of [How do I return the response from an asynchronous call?](http://stackoverflow.com/questions/14220321/how-do-i-return-the-response-from-an-asynchronous-call) — Liam, Apr 20 '17 at 08:28
Possibly better actually [Why is my variable unaltered after I modify it inside of a function? - Asynchronous code reference](http://stackoverflow.com/questions/23667086/why-is-my-variable-unaltered-after-i-modify-it-inside-of-a-function-asynchron) — Liam, Apr 20 '17 at 08:29
Asynchronous operations take an arbitrary amount of time. _Their results will not be "in order"_. — evolutionxbox, Apr 20 '17 at 08:30
using Promises you can kick off all the asyncrhonous requests, and get the results in order once the last request completes — Jaromanda X, Apr 20 '17 at 08:31

synthet1c · Accepted Answer · 2017-04-20T08:47:24.760

You can use promises and the Promise.all function to fire off a bunch of asynchronous requests and get the results in the order they were requested if all succeed. It doesn't look like you are using any of the loops values accept the index.

const readFilePromise = x => new Promise((resolve, reject) => {
  const path = Path.join(__dirname, "/../../Modified_Data.json")
  fs.readFile(path, 'utf8', (err, data) => {
    if (err) {
      return reject(err)
    }
    resolve(data)
  })
})

Promise.all(
  details.waypoints.meta.map(readFilePromise)
)
.then(data => {
  data.forEach((x, i) => console.log(x, i))
})
.catch(err => console.error(err))

Thank you very much I got the concept now – Developer Apr 20 '17 at 11:10 — Developer, Apr 20 '17 at 11:10

Loop with Asynchronous Calls

1 Answers1