实现数组方法

       // map 对数组中的每一个元素进行加工，返回加工后的新数组
        Array.prototype.myMap = function(callback) {
         let result = [];
         for(let i = 0; i < this.length; i++) {
            result.push(callback(this[i],index,this))
         }
         return result;
        }
        // forEach
        Array.prototype.myForEach = function(callback) {
            for(let i = 0; i < this.length; i++) {
            result.push(this[i],index,this)
         }
        }
        // filter
        Array.prototype.myFilter = function(callback) {
            let result = []
            for(let i = 0; i < this.length; i++) {
                if(callback(this[i],index,this)){
                    result.push(this[i])
                }
           }
         return result
        }
        //find
        Array.prototype.myFind = function(callback) {
            for(let i = 0; i < this.length; i++) {
                if(callback(this[i],index,this)){
                    return this[i]
                }
            }
            return undefined
        }
        //findIndex
        Array.prototype.myFind = function(callback) {
            for(let i = 0; i < this.length; i++) {
                if(callback(this[i],index,this)){
                    return index
                }
            }
            return undefined
        }