Array operation examples in JS

Array operations in JS

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

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, that is to say, even if the length is specified Even if the length is 5, you can still store elements beyond the specified length. Note: the length 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 a new value"; //Assign new values ​​to array elements

3. Adding array elements

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 ]]]]);// Add one or more new elements to the beginning of the array, the elements in the array will automatically move backward, and return the new length of the array

arrayObj.splice(insertPos,0,[item1[, item2[, . . . [,itemN]]]]);//Insert one or more new elements into the specified position of the array, and the elements at the insertion position will be automatically moved back ,return"".

4. Deletion of array elements

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

arrayObj.shift(); //Remove the first element and return the element value, and the elements in the array are automatically moved forward

arrayObj.splice(deletePos,deleteCount); //Delete from the specified position The specified number of deleteCount elements starting from deletePos, and the removed elements are returned in array form

5. Array interception and merging

arrayObj. slice(start, [end]); //Return a part of the array in the form of an array. Note that the elements corresponding to end are 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 new connected array Array

6. Copy of array

arrayObj.slice(0); //Return a copy array of the array, note that it is a new one Array, not pointing to

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

7, array elements Sorting

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 and return the array address

8. Stringization of the array elements

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

toLocaleString, toString, valueOf: can be regarded as special usage of join, not commonly used

2. 3 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];

//defined An array containing 10 numbers

alert(arr.length); //Display the length of the array 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, elements with index equal to or exceeding 5 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, the 9th element cannot be recovered and displays "undefined"

We can clearly see from the above code to 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);

In the code, an array containing 10 numbers is also defined first. You can use the alert statement to See 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.

For array objects, the following example illustrates the use of the prototype attribute.

Add a method to the array object that returns 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 saves the maximum value in the array x, or 6.

3. The constructor attribute

represents the function that creates the object.

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:

x = new String("Hi");

if (x.constructor == String) // Process (condition is true).

or

function MyFunc {

// Function body.

}

y = new MyFunc;

if (y.constructor == MyFunc) // Process (condition is true).

For arrays:

y = new Array();

 

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