Day05 - 2635. Apply Transform Over Each Element in Array EASY

Description❓

Given an integer array arr and a mapping function fn, return a new array with a transformation applied to each element.
The returned array should be created such that returnedArray[i] = fn(arr[i], i).
接受一個整數陣列arr和映射函數fn並且返回對原陣列進行應用轉換後的新陣列。
返回的陣列應創建為 returnedArray[i] = fn(arr[i], i)。

Please solve it without the built-in Array.map method.
請在沒有使用Array.map()的情況下解決這個問題。

Points

• for迴圈
• Array.push()
• 30Days of JS 工具書－Array.map()轉換陣列元素

Solution✍️

[ ▶️挑戰這一題 ][ 本日代碼 ]

1. 題目需要我們設計一個帶入整數陣列和映射函式，不使用array.map()，返回轉換後陣列的函式。

const map = (arr,fn)=>{
   let returnedArray = [];
   ...
   return returnedArray;
}

2. 用for迴圈遍歷 調用fn() 帶入arr陣列每個元素的值以及索引 returnedArray[i] = fn(arr[i], i)
3. 將fn()返回的值 push 到 returnedArray

const map = (arr,fn)=>{
   let returnedArray = [];
   for(let i=0;i<arr.length;i++){
      returnedArray.push(fn(arr[i],i));
   };
   return returnedArray;
}

Testcase

let arr1 = [1,2,3];
let fn1 = function plusone(n) { return n + 1; };
console.log(map(arr1,fn1));

let arr2 = [1,2,3];
let fn2 = fn = function plusI(n, i) { return n + i; }
console.log(map(arr2,fn2));

let arr3 = [10,20,30];
let fn3 = fn = function constant() { return 42; }
console.log(map(arr3,fn3));