SOURCE

// 题目：
const add = (x, y = 1) => x + y;
const mul = x => x * 3;
const div = x => x / 2;

// console.log(div(mul(add(3, 1))))
// 代码阅读性差

// 我们希望的调用方式：compose(add, mul, div)(3, 1) 
function compose(...funcs) {
    return function(...args) {
        if(funcs.length === 0) return args;
        if(funcs.length === 1) return funcs[0](...args);
        return funcs.reduce((a, b, currentIndex) => {
            if (currentIndex === 1) {
                return b(a(...args))
            } else {
                return b(a)
            }                
        })
    }
}
// let compose = (...funcs) => (...args) => {
//     if ...
// }
console.log('从左往右', compose(add, mul, div)(3, 1))

// 从后往前执行
function compose2(...funcs) {
  if (funcs.length === 0) {
    return arg => arg
  }

  if (funcs.length === 1) {
    return funcs[0]
  }

  return funcs.reduce((a, b) => (...args) => {
     return a(b(...args))
  })
}

console.log('从右往左', compose2(div, mul, add)(3))
// ==>> div(mul(add(4)))