数组

在数组开头插入元素

Array.prototype.insertFirstPosition = function(value){
    for(let i = this.length;i > 0;i--){
        this[i] = this[i-1]
    }
    this[0] = value
}

从数组开头删除元素

Array.prototype.delUndefined = function(arr){
    let newArr = []
    for(let i = 0;i < arr.length;i++){
        if(arr[i] !== undefined){
            newArr.push(arr[i])
        }
    }
    return newArr
}

Array.prototype.removeFirstPosition = function(){
    for(let i = 0;i < this.length;i++){
        this[i] = this[i+1]
    }
    return this.delUndefined(this)
}