Javascript-array objects, arrays

1. Create array mode 1. New keyword

var arr = new Array(1, 2, 3);

Mode 2. Creating an Array object using literals

var arr = [1, 2, 3];

2. Detecting whether an object is an array

1. instanceof (operator), [] instanceof Array
2. Array.isarray (), HTML5 method, with compatibility issues

3. Common methods for arrays

Specific Check MDN

// 1 栈操作(先进后出)push()pop()       //取出数组中的最后一项，修改length属性// 2 队列操作(先进先出)push()shift()     //取出数组中的第一个元素，修改length属性unshift()   //在数组最前面插入项，返回数组的长度// 3 排序方法reverse()   //翻转数组sort();     //即使是数组sort也是根据字符，从小到大排序// 带参数的sort是如何实现的？// 4 操作方法concat()    //把参数拼接到当前数组slice()     //从当前数组中截取一个新的数组，不影响原来的数组，参数start从0开始,end从1开始splice()    //删除或替换当前数组的某些项目，参数start, deleteCount, options(要替换的项目)// 5 位置方法indexOf()、lastIndexOf()   //如果没找到返回-1// 6 迭代方法 不会修改原数组(可选)every()、filter()、forEach()、map()、some()// 7 方法将数组的所有元素连接到一个字符串中。join()

4. Implementing the Array.join Method

Input: An array, such as var arr = [1, 2, 3, 4, 5]
Output:1|2|3|4|5

Version 1: string concatenation method

function myJoin(array, seperator) {  seperator = seperator || ‘,‘;  array = array || [];  if (array.length == 0){    return ‘‘;  }  var str = array[0];  for (var i = 1; i < array.length; i++) {    str += seperator + array[i];  }  return str;}var array = [6, 3, 5, 6, 7, 8, 0];console.log(myJoin(array, ‘-‘));console.log(array.join(‘-‘))

5. Implementing the Array.reverse Method

Reverses the order of the elements of a string array
Input:["A", "B", "C", "D"]
output:["D", "C", "B", "a"]

Version 1: element I and length-i-1 element interchange, half of I < array in for loop

    function myReverse(arr) {        // 数组为空即返回空数组        if (!arr || arr.length === 0) {            return [];        }        // 遍历到数组长度的一般即可        for (var i = 0; i < arr.length / 2; i++) {            var tmp = arr[i];            arr[i] = arr[arr.length - i - 1];            arr[arr.length - i -1] = tmp;        }        return arr;    }    var array = [‘a‘, ‘b‘, ‘c‘, ‘d‘];    console.log(array);    console.log(myReverse(array));    console.log(array.reverse());

6. Emptying the array

// 方式1 推荐 arr = [];// 方式2 arr.length = 0;// 方式3arr.splice(0, arr.length);

Case 1. An array of wages [1500, 1200, 2000, 2100, 1800], the deletion of wages over 2000

Mode 1: Use auxiliary arrays, traversal, and if filtering. (Disadvantage: Not in-situ operation)

var array =  [1500,1200,2000,2100,1800];var tmpArray = [];for (var i = 0; i < array.length; i++) {  if(array[i] < 2000) {    tmpArray.push(array[i]);  }}console.log(tmpArray);

Mode 2: Use the filter high-order function (functional programming)

var array =  [1500, 1200, 2000, 2100, 1800];array = array.filter(function (item, index) {  if (item < 2000) {    return true;  }  return false;});console.log(array);

Case 2. ["C", "a", "Z", "a", "X", "a"] find the position where each of a in the array appears

Mode 1. Using the Array.indexof method is also a disadvantage.

var array =  [‘c‘, ‘a‘, ‘z‘, ‘a‘, ‘x‘, ‘a‘];do {  var index = array.indexOf(‘a‘,index + 1);  if (index != -1){    console.log(index);  }} while (index > 0);

Case 3. Remove duplicate elements from an array

Mode 1. Using the K-v container, K saves the elements in the original array, and V saves the number of occurrences of the elements in the original array, regardless of the number of times it is saved in the new array.

 //new array var arr = [' C ', ' A ', ' Z ', ' a ', ' X ', ' a '];            Clear duplicate element function clear (array) {///1. The number of occurrences of an element with a key-value container var tmpdict = [];            var tmpdict = {};                for (var i = 0; i < Array.Length; i++) {var item = Array[i];                if (Tmpdict[item]) {tmpdict[item]++;                } else {Tmpdict[item] = 1; }}//2. Saved in a new array.            Elements with occurrences greater than 1 and equal to 1 in the container are added only once to the new array.            var tmparray = [];                for (var key in tmpdict) {if (tmpdict[key] = = 1) {Tmparray.push (key);                        } else {//If there are no elements of the original array in the new array, add if (tmparray.indexof (key) = =-1) {                    Tmparray.push (key);        }}} return tmparray; }//Test Console.log (Clear (arr));  

Javascript-array object, array