# 2629. Function Composition

###### tags:`Function` | `leetCode`
<font color="#01AE9A" background="E1F3F0">`easy`</font>

### 題目
Given an array of functions [f1, f2, f3, ..., fn], return a new function fn that is the function composition of the array of functions.

The function composition of [f(x), g(x), h(x)] is fn(x) = f(g(h(x))).

The function composition of an empty list of functions is the identity function f(x) = x.

You may assume each function in the array accepts one integer as input and returns one integer as output.
### Example

Input: functions = [x => x + 1, x => x * x, x => 2 * x], x = 4
Output: 65
Explanation:
Evaluating from right to left ...
Starting with x = 4.
2 * (4) = 8
(8) * (8) = 64
(64) + 1 = 65

Input: functions = [x => 10 * x, x => 10 * x, x => 10 * x], x = 1
Output: 1000
Explanation:
Evaluating from right to left ...
10 * (1) = 10
10 * (10) = 100
10 * (100) = 1000

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### 解題邏輯
這題要注意看他提示的呼叫 fn 的方式

/**
 * const fn = compose([x => x + 1, x => 2 * x])
 * fn(4) // 9
 */

因為我一開始一直不知道這個4是怎麼傳進去的，還自己亂加第二個參數ＸＤ

var compose = function(functions) {

	return function(x) {
        if(functions.length===0)return x
        for(let i = functions.length-1;i>=0;i--){
            x =functions[i](x)
        }
        return x
    }
};
 const fn = compose([x => x + 1, x => 2 * x])
  fn(4) // 9

4會在return fun 裡面接到,然後再將迴圈的初始條件設成 functions 的最後一個就可以了

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在解析中還有看到 reduceRight 的寫法蠻酷的給大家參考

function compose(functions): F {
    return (x: number) => functions.reduceRight((acc, f) => f(acc), x);
}
;