var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.unshift("Lemon","Pineapple");

Lemon,Pineapple,Banana,Orange,Apple,Mango

Array.prototype._unshift = function () {
        let arr = this
        let oldLen = this.length
        let argLen =  arguments.length
        let newLen = oldLen + argLen
        for (let i = oldLen - 1; i >= 0; i--) {
            arr[i + argLen] = arr[i]
        }
        for (let i = 0; i < argLen; i++) {
            arr[i] = arguments[i]
        }
        return newLen
    }
    let arr1 = [1, 3, 4, 5, 6]
    console.log(arr1._unshift(7, 8, 1),arr1); // 8, [7, 8, 1, 1, 3, 4, 5, 6]

 Array.prototype._push = function () {
        let arr = this
        let len = arguments.length
        for(let i = 0; i < len; i++) {
            this[this.length] = arguments[i]
        }
        return this.length
    }
    let arr = [2, 54, 65, 3, 4]

Array.prototype._reverse = function () {
        let arr = this
        let j = arr.length - 1
        for(let i = 0; i < j; i++,j--) {
            let curLeft = arr[i]
            let curRight = arr[j]
            arr[i] = curRight
            arr[j] = curLeft
        }
        return arr
    }

 let arr = []
 console.time('time1')
 for(let i = 0 ;i< 10000; i++) {
     arr.unshift(i)
 }
 console.timeEnd('time1')   // 平均耗時(shí) 5ms

    let arr = []
    console.time('time2')
    for(let j = 0 ;j< 10000; j++) {
       arr.push(j)
    }
    arr.reverse()
    console.timeEnd('time2')  // 平均耗時(shí) 0.47ms