SOURCE

function curry(fn, arr) {

    // 空数组保底
    arr = arr || []

    return function () {
        // 1. 函数参数转数组,可以用展开写法
        var args = [].slice.call(arguments)

        // 2. 之前的参数和上一次的参数用concat组合起来
        var arg = arr.concat(args)

        console.log(arg.length)

        // 3. 判断参数长度是否达到原函数参数的长度,达到了就调用函数,否则递归调用curry
        return arg.length >= fn.length
            ? fn.apply(null, arg)
            : curry(fn, arg)
    }
}

// ES6
const curry = (fn, arr = []) => (...args) => (
  arg => arg.length >= fn.length
    ? fn(...arg)
    : curry(fn, arg)
)([...arr, ...args])


function add(a, b, c, d) {
    return a + b + c + d
}

const a = curry(add)

console.log(a(1)(2, 10)(3))
console.log(a(1)(2)(3)(4))