30 Days of JS Leetcode in a nutshell

filter.js

//2634. Filter Elements from Array
//filter function under the hood
/**
 * @param {number[]} arr
 * @param {Function} fn
 * @return {number[]}
 */
var filter = function(arr, fn) {
    var a = [];
    for(let i = 0; i < arr.length; i++) {
        if(fn(arr[i], i)) {
            a.push(arr[i]);
        }
    }
    return a;
};

map.js

//2635. Apply Transform Over Each Element in Array
//map function under the the hood
/**
 * @param {number[]} arr
 * @param {Function} fn
 * @return {number[]}
 */
var map = function(arr, fn) {
    for(let i = 0; i < arr.length; ++i) {
        arr[i] = fn(arr[i], i);
    }
    return arr;
};

reduce.js

//2626. Array Reduce Transformation
//reduce function under the hood
/**
 * @param {number[]} nums
 * @param {Function} fn
 * @param {number} init
 * @return {number}
 */
var reduce = function(nums, fn, init) {
    for(let i = 0; i < nums.length; i++) {
        init = fn(init, nums[i]);
    }
    return init;
};