# [LC JS30] Closure 閉包

## Day 1 Create Hello World Function

### 題目：
Write a function createHelloWorld. It should return a new function that always returns "Hello World".
### 解答：

var createHelloWorld = function() {
    return () => "Hello World"
};

* 一般函式基本觀念：
* 具名函式
* 匿名函式(Anonymous Function)
* 立即函式(Immidiately Invoked Function Expression)
* Functions within Functions
* 函式提升(Function Hoisting)
* You can only do this if you declare functions with the `function` syntax.
* Closure
* 箭頭函式

 const f = (a, b) => {
  const sum = a + b;
  return sum;
};
console.log(f(3, 4)); // 7

### Closure 閉包
* When a function is created, it has access to a reference to all the variables declared around it, also known as it's lexical environment. The combination of the function and its enviroment is called a closure.

function createAdder(a) {
  function f(b) {
    const sum = a + b;
    return sum;
  }
  return f;
}
const f = createAdder(3);
console.log(f(4)); // 7

## Day2 Counter
### 題目：
Given an integer n, return a counter function. This counter function initially returns n and then returns 1 more than the previous value every subsequent time it is called (n, n + 1, n + 2, etc).

/** 
 * const counter = createCounter(10)
 * counter() // 10
 * counter() // 11
 * counter() // 12
 */

### 解答

var createCounter = function(n) {
    return () => n++;
};

#### ⭐知識點
* `createCounter(10)` 函式賦值到 `counter` 形成獨立的作用域，裡面的值 `n` 會重複累加
* `n++` 會先回傳值，再運算；`++n` 則會先運算再回傳值
* https://ithelp.ithome.com.tw/articles/10192800

#### 🐕自己寫的初版

var createCounter = function(n) {
    let count = 0
    return ()  => {
        count++
        return count + n - 1
    };
};

## Day3 CouterII
### 題目：
Write a function `createCounter`. It should accept an initial integer `init`. It should return an object with three functions.

The three functions are:

- `increment()` increases the current value by 1 and then returns it.
- `decrement()` reduces the current value by 1 and then returns it.
- `reset()` sets the current value to `init` and then returns it.

const counter = createCounter(5)
counter.increment(); // 6
counter.reset(); // 5
counter.decrement(); // 4

### 解答：

var createCounter = function(init) {
    let num = init
    return {
        increment: () => ++num,
        decrement: () => --num,
        reset: () => num = init
    }
};