Implement a function that takes an array of input and an async iteratee function and returns a promise that resolves with the list of inputs that has failed the test through iteratee function in JavaScript. This function is exactly the opposite of the Async Filter.
The inputs will run in parallel, but the output will be in the same order as the original.
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 = reject([1, 2, 3, 4, 5], function (num, callback) {
setTimeout(function () {
num = num * 2;
console.log(num);
// throw error
if(num === 7){
callback(true);
}else{
callback(null, num !== 4);
}
}, 2000);
});
numPromise
.then((result) => console.log("success:" + result))
.catch(() => console.log("no success"));
Output:
2
4
6
8
10
"success:2"
We can use the exact same implementation of the Async filter, only changing the logic to add the value to the output array only when it fails the test.
const reject = (arr, fn) => {
// return a new promise
return new Promise((resolve, reject) => {
const output = [];
let track = 0;
arr.forEach((e, i) => {
fn(e, (error, result) => {
// reject on error
if(error){
reject(error);
}
// track the no of inputs processed
track++;
// if input fails the test
// add it to the current index
if(!result){
output[i] = e;
}
// if the last element of the input array
if(track >= arr.length){
// filter the final output with truthy values
// to return the value in order
resolve(output.filter(Boolean));
}
});
});
});
};
let numPromise = reject([1, 2, 3, 4, 5], function (num, callback) {
setTimeout(function () {
num = num * 2;
console.log(num);
// throw error
if(num === 7){
callback(true);
}else{
callback(null, num !== 4);
}
}, 2000);
});
numPromise
.then((result) => console.log("success:" + result))
.catch(() => console.log("no success"));
Output:
2
4
6
8
10
"success:2"
