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# 題目敘述

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 1

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

# Example 2

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

# Example 3

Input: functions = [], x = 42
Output: 42
Explanation:
The composition of zero functions is the identity function

# Solution

/**
 * @param {Function[]} functions
 * @return {Function}
 */
var compose = function(functions) {
	return function(x) {
        functions.reverse().forEach(fn => x = fn(x));
        return x;
    }
};
/**
 * const fn = compose([x => x + 1, x => 2 * x])
 * fn(4) // 9
 */
type F = (x: number) => number;
function compose(functions: F[]): F {
	return function(x) {
        functions.reverse().forEach(fn => x = fn(x));
        return x;
    }
};
/**
 * const fn = compose([x => x + 1, x => 2 * x])
 * fn(4) // 9
 */