2634. Filter Elements from Array

跟前一題一樣，這題是要避免使用 Array.filter 來過濾要的元素，並回傳新的陣列

題目：

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

解題邏輯：

跟前面一樣，我們要先把函式、迴圈，跟回傳的新陣列給寫好，

 function map(arr, fn) {
 		let returnedArray=[]​
    for (let i = 0; i < arr.length; i++) {}

接著，因為要符合題目給的 Function 再取出，這次改用 if，當 Functiom 為 true 的時候再把他加到原本的 filteredArr 上。

function filter(arr, fn) {
    let filteredArr =[];
    for (let i = 0; i < arr.length; i++) {
         if (fn(arr[i], i)) {
            filteredArr.push(arr[i]) ;
            }
    }
    return filteredArr
}

完成！