Just an async mapping function in JS

test.js

const asyncMap = (array, fn, callback) => {
    var output = []; output.length = array.length;
    var counter = 0
    
    array.forEach((element, index, array) => {
        fn(element, (na, number) => {
            output[index] = number
            counter++
            
            if (output.length == counter) {
                callback(null, output)
            }
        })
    })
}
  
asyncMap([ 1, 2, 3, 4 ], (number, callback) => {
    setTimeout(() => {
        if (typeof number !== 'number') {
            callback(new Error(`Not a number: ${number}`))
            return
        }

        callback(null, number * number)
    }, Math.floor(Math.random() * 100))
}, (error, result) => {
    if (error) {
        throw error
    }
    // Should print [ 1, 4, 9, 16 ]
    console.log(result)
})