All files / src/exercises map.js

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function map(arr, fn) {
    let idx = 0,
        index = 0,
        len = arr.length,
        result = new Array(len);
 
    while (++idx < len) {
        result[index++] = fn(arr[idx - 1], idx, arr);
    }
    return result;
}
 
 
// https://stackoverflow.com/questions/8073673/how-can-i-add-new-array-elements-at-the-beginning-of-an-array-in-javascript
const array = [3, 2, 1];
const newFirstElement = 4;
const newArray = [newFirstElement].concat(array)