How to add elements to array in js unshift method

js add element unshift() to array

Add a new item to the starting position of the array:

Example

Add a new item to the starting position of the array:

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

fruits will output:

Lemon,Pineapple,Banana,Orange,Apple,Mango

Definition and usage

The unshift() method adds one or more elements to the beginning of the array and returns a new length.

Notice:This method will change the number of arrays.

hint:To add a new item to the end of the array, use the push() method.

js unshift performance analysis

js unshift() method can add elements to the head of the array and return the new array length. We handwritten an unshift() method

._unshift = function () {
        let arr = this
        let oldLen = 
        let argLen =  
        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]
    (arr1._unshift(7, 8, 1),arr1); // 8, [7, 8, 1, 1, 3, 4, 5, 6]

From the above code, we can see that first moving the element backwards and then adding elements is relatively poor in the case of a large array length.

But we can push() first and reverse() again, which is faster.

Let's handwritten push and reverse to compare

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

._reverse = function () {
        let arr = this
        let j =  - 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
    }

It can be seen that the time complexity of reverse and push is low. In actual comparison

 let arr = []
 ('time1')
 for(let i = 0 ;i&lt; 10000; i++) {
     (i)
 }
 ('time1')   // Average time consumption 5ms

    let arr = []
    ('time2')
    for(let j = 0 ;j&lt; 10000; j++) {
       (j)
    }
    ()
    ('time2')  // Average time consumption 0.47ms

Summarize

The above is personal experience. I hope you can give you a reference and I hope you can support me more.