README.md

2666. Allow One Function Call

Description

Given a function fn, return a new function that is identical to the original function except that it ensures fn is called at most once.

• The first time the returned function is called, it should return the same result as fn.
• Every subsequent time it is called, it should return undefined.

Example 1:

Input:
fn = (a,b,c) => (a + b + c), calls = [[1,2,3],[2,3,6]]
Output:
[{"calls":1,"value":6}]
Explanation:
const onceFn = once(fn);
onceFn(1, 2, 3); // 6
onceFn(2, 3, 6); // undefined, fn was not called

Example 2:

Input:
fn = (a,b,c) => (a * b * c), calls = [[5,7,4],[2,3,6],[4,6,8]]
Output:
[{"calls":1,"value":140}]
Explanation:
const onceFn = once(fn);
onceFn(5, 7, 4); // 140
onceFn(2, 3, 6); // undefined, fn was not called
onceFn(4, 6, 8); // undefined, fn was not called

Constraints:

• 1 <= calls.length <= 10
• 1 <= calls[i].length <= 100
• 2 <= JSON.stringify(calls).length <= 1000

Solution

1. 宣告一個函式once並存入函式fn做為參數。
2. 宣告內部變數wasCalled用於判斷是否曾被調用，初始值為false。
3. 以if條件式判斷wasCalled的值決定return的對象。

const once = (fn)=>{
   let wasCalled = false;

   return (...args)=>{
      if(!wasCalled){
         // wasCalled==false的處理方式
      }else{
         // wasCalled==true的處理方式
      }  
   }
}

4. wasCalled==false 時執行以下指令

• 用外部作用域的變數保存調用紀錄

  • wasCalled=true;
    調用的時候閉包會保存外部作用域變數wasCalled的值， 第一次調用判斷時初始值為false所以能成功調用。
    後續調用判斷時，由於我們將wasCalled改為true;
    這會讓條件語句進行wasCalled==true的處理方式，也就是return undefined。

  題目希望我們返回與原始函數相同結果的新函數
  做法：宣告新函式引用原始函式，同時將原始函數的參數傳遞給新函數

  • let result = fn(...args);

5. wasCalled==true 則return undefined;

const once = (fn)=>{
   let wasCalled = false;

   return (...args)=>{
      if(!wasCalled){
         wasCalled=true;

         let result = fn(...args); 
         return result;
      }else{
         return undefined;
      }  
   }
}