Detailed explanation of javascript array operations

1. Creation of array

var arrayObj = new Array(); //Create an array
var arrayObj = new Array([size]); //Create an array and specify the length, Note that it is not the upper limit, but the length.
var arrayObj = new Array([element0[, element1[, ...[, elementN]]]]); Create an array and assign a value

To explain, Although the second method creates an array and specifies the length, in fact the array is variable-length in all cases. That is to say, even if the length is specified to be 5, elements can still be stored outside the specified length. Note: At this time, the length It will change accordingly.

2. Access to the elements of the array

var testGetArrValue=arrayObj[1]; //Get the element value of the array
arrayObj[1]= "This is the new value"; / /Assign new values ​​to array elements

3. Add array elements

arrayObj. push([item1 [item2 [. . . [itemN ]]]]);// Move an or multiple new elements are added to the end of the array, and the new length of the array is returned
arrayObj.unshift([item1 [item2 [. . . [itemN ]]]]); // Add one or more new elements to the array At the beginning, the elements in the array are automatically moved backward and the new length of the array is returned
arrayObj.splice(insertPos,0,[item1[, item2[, . . . [,itemN]]]]);//Change one or more A new element is inserted into the specified position of the array, and the element at the insertion position is automatically moved back and "" is returned.

4. Deletion of array elements

arrayObj.pop(); //Remove the last element and return the element value
arrayObj.shift(); //Remove the front An element and returns the element value, the elements in the array are automatically moved forward
arrayObj.splice(deletePos,deleteCount); //Delete the specified number of deleteCount elements starting from the specified position deletePos, and return the removed elements in array form

5. Interception and merging of arrays

arrayObj.slice(start, [end]); //Return a part of the array in the form of an array. Note that the element corresponding to end is not included. If it is omitted, end will copy all elements after start
arrayObj.concat([item1[, item2[, . . . [,itemN]]]]); //Copy multiple arrays (can also be strings, or arrays and string) are connected into an array, returning the connected new array

6, copy of the array

arrayObj.slice(0); //Return the copy array of the array, Note that it is a new array, not pointing to
arrayObj.concat(); //Returns a copy array of the array. Note that it is a new array, not pointing to

7. Sorting of array elements

arrayObj.reverse(); //Reverse the elements (the first to the last, the last to the front), return the array address
arrayObj.sort(); // Sort the array elements, return the array address

8. Stringification of array elements

arrayObj.join(separator); //Return a string. This string connects each element value of the array together, with a separator in the middle. separated.
toLocaleString, toString, valueOf: can be regarded as special uses of join, not commonly used

2. Three attributes of array objects

1. Length attribute

The Length attribute represents the length of the array, that is, the number of elements in it. Because the index of an array always starts from 0, the upper and lower limits of an array are: 0 and length-1 respectively. Unlike most other languages, the length property of JavaScript arrays is variable, which requires special attention. When the length attribute is set larger, the state of the entire array does not actually change, only the length attribute becomes larger; when the length attribute is set smaller than the original, the elements in the original array with indexes greater than or equal to length will All values ​​are lost. The following is an example that demonstrates changing the length attribute:

var arr=[12,23,5,3,25,98,76,54,56,76];
//定义了一个包含10个数字的数组
alert(arr.length); //显示数组的长度10
arr.length=12; //增大数组的长度
alert(arr.length); //显示数组的长度已经变为12
alert(arr[8]); //显示第9个元素的值，为56
arr.length=5; //将数组的长度减少到5，索引等于或超过5的元素被丢弃
alert(arr[8]); //显示第9个元素已经变为"undefined"
arr.length=10; //将数组长度恢复为10
alert(arr[8]); //虽然长度被恢复为10，但第9个元素却无法收回，显示"undefined"

From the above code we can clearly see the nature of the length attribute. But the length object can not only be set explicitly, it may also be modified implicitly. You can use an undeclared variable in JavaScript. Similarly, you can also use an undefined array element (referring to an element with an index greater than or equal to length). In this case, the value of the length attribute will be set to the value of the element index used. plus 1. For example, the following code:

var arr=[12,23,5,3,25,98,76,54,56,76];
alert(arr.length);
arr[15]=34;
alert(arr.length);

The code also defines an array containing 10 numbers. It can be seen from the alert statement that its length is 10. Then the element with index 15 is used and assigned a value of 15, that is, arr[15]=34. At this time, the alert statement is used to output the length of the array, and the result is 16. Regardless, this is a surprising feature for developers accustomed to strongly typed programming. In fact, the initial length of an array created using the new Array() form is 0. It is the operation of undefined elements that causes the length of the array to change.

As you can see from the above introduction, the length attribute is so magical. You can use it to easily increase or decrease the capacity of the array. Therefore, an in-depth understanding of the length attribute will help to use it flexibly during the development process.

2. prototype attribute

Returns a reference to the prototype of the object type. The prototype property is common to object.

objectName.prototype

objectName parameter is the name of the object object.

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.

对于数组对象，以以下例子说明prototype 属性的用途。

给数组对象添加返回数组中最大元素值的方法。要完成这一点，声明一个函数，将它加入 Array.prototype， 并使用它。

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( );

该代码执行后，y 保存数组 x 中的最大值，或说 6。

3、constructor 属性

表示创建对象的函数。

object.constructor //object是对象或函数的名称。

说明：constructor 属性是所有具有 prototype 的对象的成员。它们包括除 Global 和 Math 对象以外的所有 JScript 固有对象。constructor 属性保存了对构造特定对象实例的函数的引用。

例如：

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）。

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