Implement a mapSeries async function in JavaScript that is similar to the Array.map() but returns a promise that resolves on the list of output by mapping each input through an asynchronous iteratee function or rejects it if any error occurs. The inputs are run in a sequence that is one after another.
The asynchronous iteratee function will accept an input and a callback. The callback function will be called when the input is finished processing, the first argument of the callback will be the error flag and the second will be the result.
Example
Input:
let numPromise = mapSeries([1, 2, 3, 4, 5], function (num, callback) {
// i am async iteratee function
// do async operations here
setTimeout(function () {
num = num * 2;
console.log(num);
// throw error
if(num === 12){
callback(true);
}else{
callback(null, num);
}
}, 2000);
});
numPromise
.then((result) => console.log("success:" + result))
.catch(() => console.log("no success"));
Output:
// each number will be printed after a delay of 2 seconds
2
4
6
8
10
"success:2,4,6,8,10" // this will be printed immediately after last
The implementation of this problem will be derived from the Async.series, the only difference will be that each input will be passed to the asynchronous iteratee function and the processed output from it will be returned in the output array.
We will return a new promise and inside this promise, iterate each input value in the series using the Array.reduce().
Pass the Promise.resolve([]) with an empty array as the initial accumulator to the Array.reduce() and inside the reduce, once the previous promise is resolved listen to it, create a new promise and pass the input value to the asynchronous iteratee function, based on the callback result from this iteratee function, either resolve or reject the current promise.
This will go in sequence one after another.
const mapSeries = (arr, fn) => {
// return a new promise
return new Promise((resolve, reject) => {
const output = [];
// for all the values in the input
// run it in series
// that is one after another
// initially it will take an empty array to resolve
// merge the output of the current with the previous
// and pass it on to the next
const final = arr.reduce((a, b) => {
return a.then((val) => {
// return a new promise
return new Promise((resolve, reject) => {
// based on the callback value of
// async iteratee function
// resolve or reject
fn(b, (error, result) => {
if(error){
reject(error);
}else{
resolve([...val, result]);
}
});
});
});
}, Promise.resolve([]));
// based on the final promise state
// invoke the final promise.
final
.then((result) => {
resolve(result);
})
.catch((e) => {
reject(e);
});
});
};
Test Case 1. All successful.
Input:
let numPromise = mapSeries([1, 2, 3, 4, 5], function (num, callback) {
setTimeout(function () {
num = num * 2;
console.log(num);
// throw error
if(num === 12){
callback(true);
}else{
callback(null, num);
}
}, 2000);
});
numPromise
.then((result) => console.log("success:" + result))
.catch(() => console.log("no success"));
Output:
// each number will be printed after a delay of 2 seconds
2
4
6
8
10
"success:2,4,6,8,10" // this will be printed immediately after last
Test Case 2. One fails.
Input:
let numPromise = mapSeries([1, 2, 3, 4, 5], function (num, callback) {
setTimeout(function () {
num = num * 2;
console.log(num);
// throw error
if(num === 8){
callback(true);
}else{
callback(null, num);
}
}, 2000);
});
numPromise
.then((result) => console.log("success:" + result))
.catch(() => console.log("no success"));
Output:
// each number will be printed after a delay of 2 seconds
2
4
6
8
"no success" // this will be printed immediately after last