JavaScript array operations (creation, element deletion, array copy)_Basic knowledge

1. Creation of array

Copy code The code is as follows:

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 values ​​

It should be noted that although the second method creates an array and specifies the length, in fact the array is variable-length in all cases, which means that even if the length is specified to be 5, the elements can still be stored at the specified length. Otherwise, please note: the length will change accordingly.

2. Access to array elements

Copy code The code is as follows:

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

3. Adding array elements

Copy the code The code is as follows:

arrayObj. push([item1 [item2 [. . . [itemN ]]]]); // Add one or more new elements to the end of the array and return the new length of the array
arrayObj.unshift([ item1 [item2 [. . . [itemN ]]]]);// At the beginning of adding one or more new elements to the array, the elements in the array are automatically moved back and the new length of the array is returned
arrayObj.splice(insertPos, 0,[item1[, item2[, . . . [,itemN]]]]);//Insert one or more new elements into the specified position of the array. The element at the insertion position is automatically moved back and returns "".

4. Deletion of array elements

Copy code The code is as follows:

arrayObj.pop(); //Remove the last element and return the element value
arrayObj.shift(); //Remove the first element and return 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

Copy code The code is as follows:

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 end is omitted, all elements after start will be copied
arrayObj.concat ([item1[, item2[, . . . [,itemN]]]]); //Concatenate multiple arrays (can also be strings, or a mixture of arrays and strings) into one array, and return the connected new array

6. Copy of array

Copy code The code is as follows:

arrayObj.slice(0); //Returns a 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

Copy code The code is as follows:

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

Copy code The code is as follows:

arrayObj.join(separator); //Return a string. This string connects each element value of the array together, separated by separator.

toLocaleString, toString, valueOf: can be regarded as special uses of join, which are 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:

Copy the code The code is as follows:

var arr= [12,23,5,3,25,98,76,54,56,76];
//Define an array containing 10 numbers
alert(arr.length); //Display the array The length of 10
arr.length=12; //Increase the length of the array
alert(arr.length); //Show that the length of the array has become 12
alert(arr[8]); //Display the value of the 9th element, which is 56
arr.length=5; //Reduce the length of the array to 5, and elements with indexes equal to or exceeding 5 are discarded
alert(arr[8]) ; //Show that the 9th element has become "undefined"
arr.length=10; //Restore the array length to 10
alert(arr[8]); //Although the length is restored to 10 , but the 9th element cannot be recovered and displays "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. Add 1. For example, the following code:

Copy the code The code is as follows:

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

The code also first 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 new Array() 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 attribute 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, the following example illustrates 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.

Copy code The code is as follows:

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

represents a function that creates an object.

Copy code The code is as follows:

object.constructor //object is the name of the object or function .

Description: The constructor property is a member of all objects with prototype. They include all JScript native objects except Global and Math objects. The constructor property holds a reference to the function that constructs a specific object instance.

For example:

Copy code The code is as follows:

x = new String("Hi ");
if (x.constructor == String) // Process (condition is true).

or

Copy code The code is as follows:

function MyFunc {
// function body.
}
y = new MyFunc;
if (y.constructor == MyFunc) // Process (condition is true).