Summary of the introduction to array objects in JavaScript

Array object is an array object used to store multiple values in a single variable. JS arrays are weakly typed, so the array is allowed to contain elements of different types. The array elements can even be objects or other arrays.

• Syntax for creating an array

1. Array constructor

1、var list=new Array();2、var list=new Array(size);3、var list=new Array(element0,element1,...elementn);

2. Literal method

var Array[element0,element1,...elementn];

For example

var list=new Array(1,true,null,undefined,{x:1},[1,2,3]);

var list[1,true,null,undefined,{x:1},[1,2,3]];

• Classification of arrays

1. Two-dimensional array. The essence of a two-dimensional array is that the elements in the array are is an array.

var arr = [[1,2],[a,b]]; alert(arr[1][0]); //a 第2列第1行所在的元素

2. Sparse array

A sparse array is an array containing discontinuous indexes starting from 0. In sparse arrays, the length attribute value is generally larger than the actual number of elements (uncommon)
Example
var a=["a",,"b",,,,"c",,] ;

• Array object properties

1. Length attribute

Explain the length attribute through some operations
var arr=[1,2,3,4,5,6,7,8, 9,10];//Defines an array containing 10 numbers.
The length attribute of the array is variable

alert(arr.length); //显示数组的长度10 arr.length=15; //增大数组的长度,length属性是可变的 alert(arr.length); //显示数组的长度已经变为15

Accessing the array elements

alert(arr[3]); //显示第4个元素的值，为4

Reduce the array length

arr.length=2; //将数组的长度减少到2，数组中的元素只剩下索引值小于2的元素 alert(arr[9]); //这时候显示第10个元素已经变为"undefined"因为索引值大于等于2的元素都被抛弃了

Restore the array length

arr.length=10; //将数组长度恢复为10 alert(arr[9]); //长度恢复之后已经抛弃的元素却无法收回，显示"undefined"

2. Prototype attribute

prototypeThe attribute returns a reference to the prototype of the object type.prototypeattributes are common toobject.
objectName.prototype
objectNameThe parameter is the name of theobjectobject.

Description: Use the prototype property to provide a set of basic functions of the object's class. New instances of an object "inherit" the operations assigned to the object's prototype.

For array objects, use the following example to illustrate the use of the prototype attribute.
Add a method to the array object to return the maximum element value in the array. To accomplish this, declare a function, add it to Array.prototype, and use it.

function array_max( ) { var i, max = this[0]; for (i = 1; i < this.length; i++) { if (max < this[i]) max = this[i]; } return max; }Array.prototype.max = array_max;var x = new Array(1, 2, 3, 4, 5, 6);var y = x.max( );

After this code is executed, y holds the maximum value in the array x, or say 6.

3. Constructor attribute

The constructorattribute represents the function that creates the object.
object.constructor //objectis the name of the object or function.
Description:constructorproperties are members of all objects withprototype. They include allJScriptintrinsic objects exceptGlobalandMathobjects. Theconstructorattribute holds a reference to the function that constructs a specific object instance.

For example:

x = new String("Hi");if (x.constructor == String) // 进行处理（条件为真）。//或function MyFunc {// 函数体。 } y = new MyFunc;if (y.constructor == MyFunc) // 进行处理（条件为真）。

For array:

y = new Array();

• Object method of Array

Description : Part of it is a new feature of ECMAScript5 (not supported by IE678)

var arr=[1,2,3,4,5]; arr.concat([10,11],13);//[1,2,3,4,5,10,11,13] arr.concat([1,[2,3]]);//[1,2,3,4,5,1,[1,3]]

var arr=[1,2,3,4,5]; arr.slice(1,3);//[2,3] arr.slice(1);//[2,3,4,5] arr.slice(1,-1);//[2,3,4] arr.slice(-4,-3);//[2]

var arr=[1,2,3,4,5]; arr.splice(2);//[3,4,5] arr;//[1,2];原数组被修改了 var arr=[1,2,3,4,5]; arr.splice(2，2);//[3,4] arr;//[1,2,5]; var arr=[1,2,3,4,5]; arr.splice(1，1，‘a’，‘b’);//[2] arr;//[1,"a","b",3,4,5];

var arr = [1, 2, 3, 4, 5]; arr.forEach(function(x, index, a) {//分别对应：数组元素，元素的索引，数组本身console.log(x + '|' + index + '|' + (a === arr)); });// 1|0|true// 2|1|true// 3|2|true// 4|3|true// 5|4|true

例2 var data=[1,2,3,4,5,6]; var sum=0; data.forEach(function(v){//其中的v就是数组的值 123456 sum+=v;}) document.write(sum+"
");//打印出来是21

var arr = [1, 2, 3]; arr.map(function(x) { return x + 10; }); // [11, 12, 13] arr; // [1, 2, 3]

var arr = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]; arr.filter(function(x, index) { return index % 3 === 0 || x >= 8; }); // returns [1, 4, 7, 8, 9, 10] arr; // [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

例1 every()var arr = [1, 2, 3, 4, 5]; arr.every(function(x) { return x < 10; }); // true arr.every(function(x) { return x < 3; }); // false

例2 somevar arr = [1, 2, 3, 4, 5]; arr.some(function(x) { return x === 3; }); // true arr.some(function(x) { return x === 100; }); // false

[x1, x2, x3, x4].reduce(f) = f(f(f(x1, x2), x3), x4)

var arr = [1, 2, 3];var sum = arr.reduce(function(x, y) { return x + y }, 0); //参数 0是可选的，如果写了参数0那第一次传递的两个值就是0和1 如果不写第一次传递的就是数组的前两个值，计算结果是6 arr; //[1, 2, 3] arr = [3, 9, 6];var max = arr.reduce(function(x, y) { console.log(x + "|" + y); return x > y ? x : y; });// 3|9// 9|6 max; // 9

max = arr.reduceRight(function(x, y) { console.log(x + "|" + y); return x > y ? x : y; });// 6|9// 9|3max; // 9

var arr = [1, 2, 3, 2, 1]; arr.indexOf(2); // 1 arr.indexOf(99); // -1表示没有这个元素 arr.indexOf(1, 1); // 4 arr.indexOf(1, -3); // 4查找1从倒数第3个元素开始 arr.indexOf(2, -1); // -1查找2从倒数第1个元素开始 arr.lastIndexOf(2); // 3从右边开始找第一次出现2的位置 arr.lastIndexOf(2, -2); // 3从右边的倒数第二个开始找2出现的位置 arr.lastIndexOf(2, -3); // 1

[]instanceof Array;//true ({}).toString.apply([])==='[object Array]';//true [].construcror===Array;//true