30 Days of Javascript 6 - Array Reduce Transformation

Given an integer array arr and a filtering function fn, return a filtered array filteredArr.

The fn function takes one or two arguments:

arr[i] - number from the arr i - index of arr[i] filteredArr should only contain the elements from the arr for which the expression fn(arr[i], i) evaluates to a truthy value. A truthy value is a value where Boolean(value) returns true.

Please solve it without the built-in Array.filter method.

Example 1:

Input: arr = [0,10,20,30], fn = function greaterThan10(n) { return n > 10; } Output: [20,30] Explanation: const newArray = filter(arr, fn); // [20, 30] The function filters out values that are not greater than 10 Example 2:

Input: arr = [1,2,3], fn = function firstIndex(n, i) { return i === 0; } Output: [1] Explanation: fn can also accept the index of each element In this case, the function removes elements not at index 0 Example 3:

Input: arr = [-2,-1,0,1,2], fn = function plusOne(n) { return n + 1 } Output: [-2,0,1,2] Explanation: Falsey values such as 0 should be filtered out

/**
 * @param {number[]} arr
 * @param {Function} fn
 * @return {number[]}
 */
var filter = function (arr, fn) {
  let newArray = [];
  for (let i = 0; i < arr.length; i++) {
    if (fn(arr[i], i)) {
      newArray.push(arr[i]);
    }
  }
  return newArray;
};

The filter function takes two parameters: arr, which is the original array, and fn, which is the callback function. Inside the filter function, a new array called newArray is declared to store the filtered elements. A for loop is used to iterate over each element of the arr array. Within each iteration of the loop, the callback function fn is called with two arguments: the current element of the arr array (arr[i]) and the index of that element (i). The result of the callback function is checked with an if statement. If the result is truthy (e.g., true, a non-zero number, a non-empty string), the current element is pushed into the newArray. After iterating over all elements of the arr array, the newArray is returned as the result of the filter function.