SOURCE

const __ = Symbol('PLACEHOLDER');

function placehoder(fn, ...bound){
    return function newFn(...args){
        // 先按函数的参数长度补齐占位
        while(bound.length < fn.length){
            bound.push(__)
        }
        // 替换参数        
        let j = 0; 


        for(let i = 0; i < fn.length; i++){
            if(bound[i] === __){
                if(j < args.length){
                    bound[i] = args[j]
                    j++
                }
            }
        }
        // 没有占位符了执行
        if(bound.indexOf(__) == -1 && bound.length >= fn.length){
            // console.log(bound)
            return fn.apply(this, bound)
        }else{
            // 返回一个方法接收新的参数
            return (...args1)=>{
                return newFn.apply(this, args1)
            }
        }
    }
}

function dot(x1, y1, x2, y2){
    return x1 * x2 + y1 * y2
}

console.log(placehoder(dot, 10, __ , 10, __)(2, __)(3))
console.log(dot(10, 2, 10, 3))

console.log(placehoder(dot, __, __ , __, 5)(4, __, 2)(__)(3))
console.log(dot(4, 3, 2, 5))

console.log(placehoder(dot,3)(__, __)(4, __, 2)(3))
console.log(dot(3, 4, 3, 2))

console.log(placehoder(dot)(3, __, __, 4)(5, 6))
console.log(dot(3, 5, 6, 4))


console.log(placehoder(dot, 3,4, 5, 3)())
console.log(dot(3, 4, 5, 3))