Operations on js Arrays

I have been using js for a long time, but I have never gotten into the js array form. Occasionally, it is a simple string. split (char ). During this period of time, there were a lot of projects that used arrays. I thought I could not help myself as a js master! Haha. After learning it, I learned that the js array is far more powerful than VB and C #. Let's take a look.

1. Create an array

Copy codeCode: var arrayObj = new Array (); // create an Array
Var arrayObj = new Array ([size]); // create an Array and specify the length. Note that the length is not the upper limit.
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 with a specified length, the array actually gets longer in all cases, that is, even if the length is 5, you can still store elements outside the specified length. Note: The length will change accordingly.

2. Access to array elements

Copy codeCode: var testGetArrValue = arrayObj [1]; // obtain the element value of the array.
ArrayObj [1] = "this is a new value"; // assign a new value to the array element

3. Add array elements

Copy codeThe Code is as follows: arrayObj. push ([item1 [item2 [... [itemN]); // adds one or more new elements to the end of the array and returns the new length of the array.
ArrayObj. unshift ([item1 [item2 [... [itemN]); // adds one or more new elements to the array. The elements in the array are automatically removed and the new length of the array is returned.
ArrayObj. splice (insertPos, 0, [item1 [, item2 [,... [, itemN]); // insert one or more new elements to the specified position of the array. The inserted element is automatically removed and "" is returned "".

4. Deletion of array elements

Copy codeThe Code is as follows: arrayObj. pop (); // removes the last element and returns the value of this element.
ArrayObj. shift (); // removes the first element and returns the element value. The elements in the array are automatically moved forward.
ArrayObj. splice (deletePos, deleteCount); // deletes the specified number of deleteCount elements starting from deletePos in the specified position. The removed elements are returned in the array format.

5. truncate and merge Arrays

Copy codeThe Code is as follows: arrayObj. slice (start, [end]); // return part of the array in the form of an array. Note that the elements corresponding to the end are not included. If the end is omitted, all elements after the start are copied.
ArrayObj. concat ([item1 [, item2 [,... [, itemN]); // concatenates multiple arrays (or strings, or arrays and strings) into an array and returns a new connected array.

6. Copy an array

Copy codeThe Code is as follows: arrayObj. slice (0); // returns the copy array of the array. Note that it is a new array, not pointing
ArrayObj. concat (); // returns the copy array of the array. Note that it is a new array, not pointing

7. Sorting of array elements

Copy codeThe Code is as follows: arrayObj. reverse (); // returns the array address.
ArrayObj. sort (); // sorts array elements and returns the array address.

8. stringized array elements

Copy codeThe Code is as follows: arrayObj. join (separator); // returns a string that connects each element value of the array and is separated by separator.
ToLocaleString, toString, valueOf: can be considered as a special use of join, not commonly used

2. Three attributes of an array object

1. length attribute

The Length attribute indicates the Length of the array, that is, the number of elements. Because the index of an array always starts from 0, the upper and lower limits of an array are: 0 and length-1. Unlike most other languages, the length attribute of the JavaScript array is variable, which requires special attention. When the length attribute is set to a greater value, the status of the entire array does not actually change, except that the length attribute becomes larger. When the length attribute is set to an hour later than the original value, all the values of the elements whose indexes are greater than or equal to the length in the original array are lost. The following is an example of changing the length attribute:

Copy codeCode: var arr = [,];
// Defines an array containing 10 numbers
Alert (arr. length); // display the length of the array by 10
Arr. length = 12; // increase the length of the array.
Alert (arr. length); // display that the length of the array has changed to 12
Alert (arr [8]); // displays the value of the 9th elements, 56
Arr. length = 5; // reduce the length of the array to 5. elements whose index is equal to or greater than 5 are discarded.
Alert (arr [8]); // display that 9th elements have changed to "undefined"
Arr. length = 10; // restore the array length to 10
Alert (arr [8]); // although the length is restored to 10, 9th elements cannot be recovered, and "undefined" is displayed"

From the code above, we can clearly see the nature of the length attribute. However, the length object can be explicitly set and may be implicitly modified. JavaScript can use an undeclared variable, or an undefined array element (an element whose index exceeds or is equal to length, the value of the length attribute is set to the value of the used element index plus 1. For example, the following code:

Copy codeCode: var arr = [,];
Alert (arr. length );
Arr [15] = 34;
Alert (arr. length );

The Code also defines an array containing 10 numbers. The alert statement shows that the length is 10. Then, an element with an index of 15 is used and assigned a value of 15, that is, arr [15] = 34. Then, the array length is output using the alert statement, and the result is 16. In any case, this is a surprising feature for developers who are used to strong-type programming. In fact, the initial length of an Array created in the form of new Array () is 0, and the length of the Array changes only when no element is defined.
From the above introduction, we can see that the length attribute is so magical that it can be used to conveniently increase or decrease the array capacity. Therefore, an in-depth understanding of the length attribute can be used flexibly in the development process.

2. prototype attributes

Returns a reference to an object type prototype. The prototype attribute is common to objects.
ObjectName. prototype
The objectName parameter is the name of the object.

Note: The prototype attribute is used to provide a set of basic functions of the object class. The new instance of the object "inherits" the operation that is granted to the object prototype.
The following example describes the purpose of the prototype attribute for array objects.
Add a method to the array object to return the maximum element value in the array. To do this, declare a function, add it to Array. prototype, and use it.

Copy codeThe 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 the code is executed, y saves the maximum value in array x, or 6.

3. constructor attributes

The function that creates an object.
Object. constructor // object is the name of an object or function.

Note: The constructor attribute is a member of all objects with prototype. They include all inherent JScript objects except Global and Math objects. The constructor attribute stores references to the functions used to construct a specific object instance.

For example:

Copy codeThe Code is as follows: x = new String ("Hi ");
If (x. constructor = String) // process (the condition is true ).

Or

Copy codeThe Code is as follows: function MyFunc {
// Function body.
}

Copy codeThe Code is as follows: y = new MyFunc;
If (y. constructor = MyFunc) // process (the condition is true ).

For arrays:

Copy codeThe Code is as follows: y = new Array ();