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{one week summary} take you into the ocean of JS knowledge

2022-07-06 11:28:00 【Geek Yunxi】

Catalog

JavaScript Output

JavaScript Display the data

Use window.alert()

example

operation HTML Elements

JavaScript Code block

example

JavaScript Statement identifier

Statement （ establish ） JavaScript Variable

example

A sentence , Multiple variables

Value = undefined

To declare JavaScript Variable

JavaScript Count

example

JavaScript data type

JavaScript Have dynamic types

example

JavaScript character string

Declare variable type

JavaScript object

Objects in real life , Properties and methods

Access object properties

Access object methods

example

JavaScript function

example

JavaScript Function syntax

Call function with parameters

example

Function with return value

grammar

example

Local JavaScript Variable

overall situation JavaScript Variable

JavaScript Lifetime of variables

To undeclared JavaScript Variable assignment value

JavaScript What can be done ?

JavaScript character string

Strings can be objects

example

String properties and methods

String properties

String method

JavaScript Math（ Count ） object

JavaScript Array（ Array ） object

You can have different objects in an array

Array methods and properties

Create new methods

example ： Create a new method .

JavaScript Output

JavaScript There are no print or output functions .

JavaScript Display the data

JavaScript You can output data in different ways ：

Use window.alert() Pop up warning box .
Use document.write() Method to write the content to HTML In the document .
Use innerHTML Write to HTML Elements .
Use console.log() Write to the browser's console .

Use window.alert()

You can pop up a warning box to display the data ：

<!DOCTYPE html>
<html>
<body>

<h1> My first page </h1>
<p> My first paragraph .</p>

<script>
window.alert(5 + 6);
</script>

</body>
</html>

Try it »

operation HTML Elements

From JavaScript Visit a HTML Elements , You can use document.getElementById(id) Method .

Please use "id" Property to identify HTML Elements , and innerHTML To get or insert element content ：

<!DOCTYPE html>
<html>
<body>

<h1> My first one  Web  page </h1>

<p id="demo"> My first paragraph </p>

<script>
document.getElementById("demo").innerHTML = " Paragraph modified .";
</script>

</body>
</html>

Try it »

above JavaScript sentence （ stay <script> In the label ） Can be in web In the browser ：

document.getElementById("demo") It's using id Property to find HTML Elemental JavaScript Code .

innerHTML = " Paragraph modified ." Is used to modify elements HTML Content (innerHTML) Of JavaScript Code .

In this tutorial

in the majority of cases , In this tutorial , We will use the method described above to output ：

The above example directly puts id="demo" Of <p> Element write HTML Document output in progress ：

writes HTML file

For testing purposes , You can use JavaScript Write directly in HTML In the document ：

<!DOCTYPE html>
<html>
<body>

<h1> My first one  Web  page </h1>

<p> My first paragraph .</p>

<script>
document.write(Date());
</script>

</body>
</html>

Try it »

	Please use document.write() Just output what you write to the document . If you execute after the document has finished loading document.write, Whole HTML The page will be covered .

Please use document.write() Just output what you write to the document .

If you execute after the document has finished loading document.write, Whole HTML The page will be covered .

!DOCTYPE html>
<html>
<body>

<h1> My first one  Web  page </h1>

<p> My first paragraph .</p>

<button onclick="myFunction()"> Am I </button>

<script>
function myFunction() {
    document.write(Date());
}
</script>

</body>
</html>

Try it »

Write to the console

If your browser supports debugging , You can use console.log() Method is displayed in the browser JavaScript value .

The browser uses F12 To enable debug mode , Click... In the debug window "Console" menu .

<!DOCTYPE html>
<html>
<body>

<h1> My first one  Web  page </h1>

<script>
a = 5;
b = 6;
c = a + b;
console.log(c);
</script>

</body>
</html>

Try it »

example console Screenshot ：

JavaScript sentence

JavaScript Statement to the browser . The purpose of the statement is to tell the browser what to do .

JavaScript sentence

JavaScript Statement is a command to the browser .
The purpose of these commands is to tell the browser what to do .
Below JavaScript Statement to id="demo" Of HTML Element output text " Hello Dolly" ：

document.getElementById("demo").innerHTML = " Hello  Dolly";

Try it »

A semicolon ;

Semicolons are used to separate JavaScript sentence .

Usually we add a semicolon at the end of each executable statement .

Another use of semicolons is to write multiple statements on a line .

a = 5; b = 6; c = a + b;

 The above example can also be written like this :

a = 5; b = 6; c = a + b;

Try it »

	You may also see cases without semicolons . stay JavaScript in , Ending a statement with a semicolon is optional .

JavaScript Code

JavaScript The code is JavaScript Sequence of statements .

The browser executes each statement in writing order .

This example outputs a title and two paragraphs to the web page ：

example

document.getElementById("demo").innerHTML=" Hello  Dolly"; document.getElementById("myDIV").innerHTML=" How are you doing? ?";

Try it »

JavaScript Code block

JavaScript Can be combined in batches .

The code block starts with an open curly bracket , End with a closing curly bracket .

The function of a code block is to execute a sequence of statements together .

This example outputs a title and two paragraphs to the web page ：

example

function myFunction() { document.getElementById("demo").innerHTML=" Hello Dolly"; document.getElementById("myDIV").innerHTML=" How are you doing? ?"; }

Try it »

You will learn more about functions in later chapters .

JavaScript Statement identifier

JavaScript A statement usually takes the form of a Statement identifier For the beginning , And execute the statement .

Statement identifiers are reserved keywords and cannot be used as variable names .

The following table lists them JavaScript Statement identifier ( keyword ) ：

sentence	describe
break	For jumping out of the loop .
catch	Sentence block , stay try When a statement block fails to execute catch Sentence block .
continue	Skip an iteration in the loop .
do ... while	Execute a statement block , When the condition statement is true Continue to execute the statement block .
for	When the condition statement is true when , The code block can be executed a specified number of times .
for ... in	Properties used to traverse arrays or objects （ Loop through the properties of an array or object ）.
function	Define a function
if ... else	Used to perform different actions based on different conditions .
return	Exit function
switch	Used to perform different actions based on different conditions .
throw	Throw out （ Generate ） error .
try	Implement error handling , And catch Use together .
var	Declare a variable .
while	When the conditional statement is true when , Execute statement block .

Space

JavaScript Will ignore extra spaces . You can add spaces to the script , To improve its readability . The following two lines of code are equivalent ：

var person="runoob";
var person = "runoob";

Break lines of code

You can use backslashes in text strings to wrap lines of code . The following example will correctly show ：

document.write(" Hello  \  The world !");

Try it »

however , You can't fold like this ：

document.write \ 
(" Hello world !");

Knowledge point ：JavaScript It's script language , When the browser reads the code , Execute script code line by line . For traditional programming , All code will be compiled before execution .

JavaScript Variable

Variables are used to store information " Containers ".

var x=5; var y=6; var z=x+y;

Try it »

It's like algebra

x=5
y=6
z=x+y

In algebra , We use letters （ such as x） To save value （ such as 5）.

Through the expression above z=x+y, We can calculate z The value of is 11.

stay JavaScript in , These letters are called variables .

	You can think of variables as containers for storing data .

JavaScript Variable

Same as algebra ,JavaScript Variables can be used to store values （ such as x=5） And expressions （ such as z=x+y）.

Variables can have short names （ such as x and y）, You can also use better descriptive names （ such as age, sum, totalvolume）.

Variable must start with a letter
Variables can also be $ and _ Symbol at the beginning （ But we don't recommend it ）
Variable names are case sensitive （y and Y It's a different variable ）

	JavaScript Statement and JavaScript Variables are case sensitive .

JavaScript data type

JavaScript Variables can also hold other data types , For example, text value (name="Bill Gates").
stay JavaScript in , similar "Bill Gates" Such a text is called a string .
JavaScript There are many types of variables , But now , We only focus on numbers and strings .
When you assign a text value to a variable , You should enclose this value in double or single quotes .
When the value you assign to a variable is a numeric value , Do not use Quotes . If you enclose values in quotation marks , The value is treated as text .

var pi=3.14; //  If you are familiar with  ES6,pi  have access to  const  keyword , Represents a constant  // const pi = 3.14; var person="John Doe"; var answer='Yes I am!';

Try it »

Statement （ establish ） JavaScript Variable

stay JavaScript Creating variables in is often called " Statement " Variable .

We use var Keywords to declare variables ：

var carname;

After variable declaration , The variable is empty （ It's not worth it ）.

To assign a value to a variable , Please use the equal sign ：

carname="Volvo";

however , You can also assign values to variables when declaring them ：

var carname="Volvo";

In the following example , We've created something called carname The variable of , And assign to it "Volvo", Then put it in id="demo" Of HTML Duan Luozhong ：

var carname="Volvo"; document.getElementById("demo").innerHTML=carname;

Try it »

	A good programming habit is , At the beginning of the code , Uniformly declare the required variables .

A sentence , Multiple variables

You can declare many variables in one statement . This statement takes var start , And use comma to separate variables ：

var lastname="Doe", age=30, job="carpenter";

Statements can also span multiple lines ：

var lastname="Doe",
age=30,
job="carpenter";

Multiple variables declared in a statement cannot be assigned the same value at the same time :

var x,y,z=1;

x,y  by  undefined, z  by  1.

Value = undefined

In a computer program , Variables with no value are often declared . Variables declared without values , The value is actually undefined.

After executing the following statement , Variable carname The value will be undefined：

var carname;

To declare JavaScript Variable

If you restate JavaScript Variable , The value of this variable will not be lost ：

After the following two statements are executed , Variable carname The value of PI is still zero "Volvo"：

var carname="Volvo";
var carname;

JavaScript Count

You can JavaScript Variable to calculate , It uses = and + Such operators ：

y=5; x=y+2;

Try it »

JavaScript data type

Value type ( Basic types )： character string （String）、 Numbers (Number)、 Boolean (Boolean)、 empty （Null）、 Undefined （Undefined）、Symbol.

Reference data type （ object type ）： object (Object)、 Array (Array)、 function (Function), There are two special objects ： Regular （RegExp） And the date （Date）.

notes ：Symbol yes ES6 A new type of raw data is introduced , Represents a unique value .

JavaScript Have dynamic types

JavaScript Have dynamic types . This means that the same variable can be used for different types ：

var x;               // x  by  undefined
var x = 5;           //  Now?  x  Is the number 
var x = "John";      //  Now?  x  For the string

The data type of the variable can use typeof Operator to see ：

typeof "John"                //  return  string
typeof 3.14                  //  return  number
typeof false                 //  return  boolean
typeof [1,2,3,4]             //  return  object
typeof {name:'John', age:34} //  return  object

Try it »

JavaScript character string

Strings are stored characters （ such as "Bill Gates"） The variable of .

A string can be any text in quotation marks . You can use single quotes or double quotes ：

var carname="Volvo XC60";
var carname='Volvo XC60';

You can use quotation marks in strings , Just don't match the quotation marks surrounding the string ：

var answer="It's alright";
var answer="He is called 'Johnny'";
var answer='He is called "Johnny"';

Try it »

You will learn more about strings in the advanced part of this tutorial .

JavaScript Numbers

JavaScript There is only one type of number . Numbers can have decimal points , It's also possible to do without ：

var x1=34.00;      // Use the decimal point to write 
var x2=34;         // Don't use the decimal point to write

The maximum or minimum number can be determined by science （ Index ） Count to write ：

var y=123e5;      // 12300000
var z=123e-5;     // 0.00123

Try it »

You will learn more about numbers in the advanced part of this tutorial .

JavaScript Boolean

Boolean （ Logic ） There are only two values ：true or false.

var x=true;
var y=false;

Boolean is often used in conditional testing . You'll learn more about conditional testing later in this tutorial .

JavaScript Array

The following code creates a code named cars Array of ：

var cars=new Array();
cars[0]="Saab";
cars[1]="Volvo";
cars[2]="BMW";

 perhaps  (condensed array):

var cars=new Array("Saab","Volvo","BMW");

 perhaps  (literal array):

var cars=["Saab","Volvo","BMW"];

Try it »

Array subscripts are zero based , So the first project is [0], The second is [1], And so on .

You will learn more about arrays later in this tutorial .

JavaScript object

Objects are separated by curly braces . Inside the brackets , The properties of an object are in the form of name and value pairs (name : value) To define . Attributes are separated by commas ：

var person={firstname:"John", lastname:"Doe", id:5566};

 The object in the example above  (person)  There are three properties ：firstname、lastname  as well as  id.

 Spaces and breaks don't matter . Declarations can span multiple lines ：

var person={
firstname : "John",
lastname  : "Doe",
id        :  5566
};

There are two ways to address object properties ：

example

name=person.lastname;
name=person["lastname"];

Try it »

You will learn more about objects later in this tutorial .

Undefined and Null

Undefined This value means that the variable has no value .

You can do this by setting the value of the variable to null To clear variables .

example

cars=null;
person=null;

Try it »

Declare variable type

When you declare a new variable , You can use keywords "new" To declare its type ：

var carname=new String;
var x=      new Number;
var y=      new Boolean;
var cars=   new Array;
var person= new Object;

	JavaScript Variables are objects . When you declare a variable , It creates a new object .

JavaScript object

JavaScript Objects are data that have properties and methods .

Objects in real life , Properties and methods

In real life , A car is an object .

Object has its properties , Such as weight and color , Methods include start, stop, etc :

object	attribute	Method
	car.name = Fiat car.model = 500 car.weight = 850kg car.color = white	car.start() car.drive() car.brake() car.stop()

All cars have these properties , But the properties of each car are different .

All cars have these methods , But they are executed at different times .

JavaScript object

stay JavaScript in , Almost everything is an object .

	stay JavaScript in , Objects are very important , When you understand the object , You can understand JavaScript .

You have learned JavaScript The assignment of a variable .

The following code is a variable car Set the value to "Fiat" :

var car = "Fiat";

Object is also a variable , But objects can contain multiple values （ Multiple variables ）, Each value is expressed in name:value Pair presentation .

var car = {name:"Fiat", model:500, color:"white"};

In the above example ,3 It's worth ("Fiat", 500, "white") Assign variable car.

	JavaScript Objects are containers of variables .

Object definitions

You can use characters to define and create JavaScript object :

example

var person = {firstName:"John", lastName:"Doe", age:50, eyeColor:"blue"};

Try it »

Definition JavaScript Objects can span multiple lines , Spaces and line breaks are not required ：

example

var person = {
    firstName:"John",
    lastName:"Doe",
    age:50,
    eyeColor:"blue"
};

Try it »

Object properties

so to speak "JavaScript Objects are containers of variables ".
however , We usually think that "JavaScript Objects are containers for key value pairs ".
Key value pairs are usually written as name : value ( Keys and values are separated by colons ).
Key value pairs are in JavaScript Objects are often called Object properties .

	JavaScript Objects are containers for attribute variables .

Object key value pairs are written similar to ：

PHP The associative array in the
Python Dictionary in
C Hash table in language
Java Hash mapping in
Ruby and Perl Hash table in

Access object properties

You can access object properties in two ways :

example 1

person.lastName;

Try it »

example 2

person["lastName"];

Try it »

Object methods

Object defines a function , And stored as an attribute of the object .

Object method by adding () call ( As a function ).

This instance accesses person Object's fullName() Method :

example

name = person.fullName();

Try it »

If you want to visit person Object's fullName attribute , It will be returned as a string defining the function ：

example

name = person.fullName;

Try it »

	JavaScript Objects are containers for properties and methods .

In the following tutorial, you will learn more about functions , Knowledge of attributes and methods .

Access object methods

You can use the following syntax to create object methods ：

methodName : function() {
    //  Code  
}

You can access object methods using the following syntax ：

example

objectName.methodName()

Try it »

Usually fullName() As a person A method of an object , fullName As an attribute .

If you use fullName attribute , Don't add (), It will return the definition of the function ：

example

objectName.methodName

Try it »

There are many ways to create , Use and modify JavaScript object .

There are also many ways to create , Use and modify properties and methods .

JavaScript function

A function is an event driven or reusable block of code that executes when it is called .

example

<!DOCTYPE html> 
<html> 
<head> 
<meta charset="utf-8"> 
<title> Test case </title> 
<script> 
function myFunction() { alert("Hello World!"); } 
</script> 
</head> 
<body> 
<button onclick="myFunction()"> Am I </button> 
</body> 
</html>

Try it »

JavaScript Function syntax

Functions are blocks of code wrapped in curly braces , Key words used earlier function：

function functionname()
{
    //  Execute code 
}

When this function is called , Will execute code within the function .

Functions can be called directly when an event occurs （ For example, when the user clicks the button ）, And can be JavaScript Call anywhere .

	JavaScript Case sensitive . key word function Must be lowercase , And the function must be called in the same case as the function name .

Call function with parameters

When the function is called , You can pass values to , These values are called parameters .
These parameters can be used in functions .
You can send as many parameters as you want , Comma (,) Separate ：

myFunction(argument1,argument2)

 When you declare a function , Please declare the parameter as a variable ：

function myFunction(var1,var2)
{
 Code 
}

Variables and parameters must appear in the same order . The first variable is the given value of the first parameter passed , And so on .

example

<p> Click this button , To call a function with parameters .</p> <button onclick="myFunction('Harry Potter','Wizard')"> Click here </button> <script> function myFunction(name,job){ alert("Welcome " + name + ", the " + job); } </script>

Try it »

The above function will prompt when the button is clicked "Welcome Harry Potter, the Wizard".

Functions are flexible , You can call this function with different parameters , This will give a different message ：

example

<button onclick="myFunction('Harry Potter','Wizard')"> Click here </button> <button onclick="myFunction('Bob','Builder')"> Click here </button>

Try it »

According to the different buttons you click , The above example will prompt "Welcome Harry Potter, the Wizard" or "Welcome

Bob, the Builder".

Function with return value

Sometimes , We want the function to return the value to where it was called .

By using return Statement can be implemented .

In the use of return When the sentence is , Function will stop executing , And returns the specified value .

grammar

function myFunction()
{
    var x=5;
    return x;
}

The above function will return a value 5.

Be careful ： Whole JavaScript Does not stop execution , Just a function .JavaScript Will continue to execute the code , From where the function is called .

The function call will be replaced by the return value ：

var myVar=myFunction();

myVar The value of the variable is 5, That's the function "myFunction()" The value returned .

Even if you don't save it as a variable , You can also use the return value ：

document.getElementById("demo").innerHTML=myFunction();

"demo" Elemental innerHTML Will become 5, That's the function "myFunction()" The value returned .

You can base the return value on the parameters passed into the function ：

example

Calculate the product of two numbers , And return the result ：

function myFunction(a,b) { return a*b; } document.getElementById("demo").innerHTML=myFunction(4,3);

"demo"  Elemental  innerHTML  It will be ：

12

Try it »

When you just want to exit the function , You can also use return sentence . The return value is optional ：

function myFunction(a,b) { if (a>b) { return; } x=a+b }

If a Greater than b, The above code will exit the function , It doesn't count a and b The sum of .

Local JavaScript Variable

stay JavaScript Variables declared within a function （ Use var） yes Local Variable , So you can only access it inside a function .（ The scope of this variable is local ）.

You can use local variables with the same name in different functions , Because only the function that has declared the variable can recognize the variable .

As long as the function runs , Local variables will be deleted .

overall situation JavaScript Variable

Variables declared outside functions are overall situation Variable , All scripts and functions on the web page can access it .

JavaScript Lifetime of variables

JavaScript The lifetime of variables begins at the time they are declared .

Local variables are removed after the function runs .

Global variables are deleted after the page is closed .

To undeclared JavaScript Variable assignment value

If you assign a value to an undeclared variable , This variable will be automatically treated as window A property of .

This statement ：

carname="Volvo";

 Will declare  window  A property of  carname.

Global variables created by assigning values to undeclared variables in non strict mode , Is a configurable property of a global object , You can delete .

var var1 = 1; //  Global properties are not configurable 
var2 = 2; //  Not used  var  Statement , Configurable global properties 

console.log(this.var1); // 1
console.log(window.var1); // 1
console.log(window.var2); // 2

delete var1; // false  Cannot delete 
console.log(var1); //1

delete var2; 
console.log(delete var2); // true
console.log(var2); //  Has deleted   Error reporting variable not defined

JavaScript event

HTML The incident happened in HTML Things on the element .

When in HTML Page usage JavaScript when , JavaScript These events can be triggered .

HTML event

HTML Events can be browser behavior , It can also be user behavior .

Here are HTML Instances of events ：

HTML Page load complete
HTML input When the field changes
HTML Button clicked

Usually , When the event occurs , You can do something .

When the event is triggered JavaScript Some code can be executed .

HTML Event attributes can be added to the element , Use JavaScript Code to add HTML Elements .

Single quotation marks :

<some-HTML-element some-event='JavaScript  Code '>

Double quotes :

<some-HTML-element some-event="JavaScript Code ">

In the following example , Button element added onclick attribute ( And add the code ):

example

<button οnclick="getElementById('demo').innerHTML=Date()"> The time is now ?</button>

Try it »

In the example above ,JavaScript Code will be modified id="demo" Content of element .

In the next instance , The code will modify the contents of its own elements ( Use this.innerHTML):

example

<button οnclick="this.innerHTML=Date()"> The time is now ?</button>

Try it »

	JavaScript Code is usually a few lines of code . It is more common to call... Through event properties ：

example

<button οnclick="displayDate()"> The time is now ?</button>

Try it »

common HTML event

Here are some common HTML List of events :

event	describe
onchange	HTML Element change
onclick	The user clicks HTML Elements
onmouseover	Occurs when the mouse pointer moves over the specified element
onmouseout	User from a HTML Occurs when you move the mouse away from the element
onkeydown	User presses keyboard key
onload	Browser finished loading page

More event lists : JavaScript Reference manual - HTML DOM event .

JavaScript What can be done ?

Events can be used to process form validation , User input , User behavior and browser action :

Event triggered on page load
Event triggered when the page is closed
The user clicks the button to perform the action
Verify the validity of user input
wait ...

There are many ways to do this JavaScript Event code ：

HTML Event properties can be executed directly JavaScript Code
HTML Event properties can be called JavaScript function
You can have the HTML Element specifies its own event handler
You can stop things from happening .
wait ...

JavaScript character string

JavaScript Strings are used to store and process text .

JavaScript character string

Strings can store a series of characters , Such as "John Doe".

A string can be any character inserted into quotation marks . You can use single quotes or double quotes ：

example

var carname = "Volvo XC60";
var carname = 'Volvo XC60';

You can use the index position to access each character in the string ：

example

var character = carname[7];

The index of the string is from 0 Start , This means that the first character index value is [0], The second is [1], And so on .
You can use quotation marks in strings , The quotation marks in the string should not be the same as those in the string :

example

var answer = "It's alright";
var answer = "He is called 'Johnny'";
var answer = 'He is called "Johnny"';

You can also use quotation marks by adding escape characters to the string ：

example

var x = 'It\'s alright';
var y = "He is called \"Johnny\"";

Try it »

String length

You can use built-in properties length To calculate the length of a string ：

example

var txt = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
var sln = txt.length;

Try it »

Special characters

stay JavaScript in , Strings are written in single or double quotation marks .

Because of this , The following examples JavaScript Unable to resolve ：

"We are the so-called "Vikings" from the north."

character string "We are the so-called " Truncated .

How to solve the above problems ？ You can use backslashes (\) To escape "Vikings" Double quotes in string , as follows :

 "We are the so-called \"Vikings\" from the north."

The backslash is a Escape character . Escape character converts a special character to a string character ：
Escape character (\) Can be used to escape apostrophes , Line break , quotes , And so on .
The following table lists the special characters that can be escaped with escape characters in a string ：

Code	Output
\'	Single quotation marks
\"	Double quotes
\\	The backslash
\n	Line break
\r	enter
\t	tab( tabs )
\b	Back space
\f	Page identifier

Strings can be objects

Usually , JavaScript The string is the original value , You can use characters to create ： var firstName = "John"

But we can also use new Keyword defines a string as an object ： var firstName = new String("John")

example

var x = "John";
var y = new String("John");
typeof x //  return  String
typeof y //  return  Object

Try it »

	Don't create String object . It slows down execution , And may have other side effects ：

example

var x = "John";             
var y = new String("John");
(x === y) //  The result is  false, because  x  Is string ,y  It's the object

Try it »

=== Is absolute equality , That is, the data type and value must be equal .

String properties and methods

The original value string , Such as "John", There are no properties or methods ( Because they're not objects ).

The original value can be used JavaScript Properties and methods of , because JavaScript When executing methods and properties, you can treat the original value as an object .

The string method will be introduced in the next chapter .

String properties

attribute	describe
constructor	Returns the function that creates the string property
length	Returns the length of the string
prototype	Allows you to add properties and methods to an object

String method

For more method examples, see ：JavaScript String object .

Method	describe
charAt()	Returns the character at the specified index position
charCodeAt()	Returns the character at the specified index position Unicode value
concat()	Concatenate two or more strings , Return the connected string
fromCharCode()	take Unicode Convert to string
indexOf()	Returns the location of the first occurrence of a specified character in a string
lastIndexOf()	Returns the position of the last occurrence of the specified character in a string
localeCompare()	Compare two strings in a local specific order
match()	Find a match for one or more regular expressions
replace()	Replace substrings that match regular expressions
search()	Retrieve the value that matches the regular expression
slice()	Extract a fragment of a string , And return the extracted part in the new string
split()	Split a string into a substring array
substr()	Extract the specified number of characters in the string from the starting index number
substring()	Extract the character between two specified index marks in the string
toLocaleLowerCase()	Convert the string to lowercase according to the host language environment , There are only a few languages （ Like Turkish ） With local case mapping
toLocaleUpperCase()	Convert the string to uppercase according to the host language environment , There are only a few languages （ Like Turkish ） With local case mapping
toLowerCase()	Convert string to lowercase
toString()	Return string object value
toUpperCase()	Convert strings to uppercase
trim()	Remove white space at the beginning and end of a string
valueOf()	Returns the original value of a string object

JavaScript Math（ Count ） object

Math（ Count ） The object's function is ： Perform common arithmetic tasks .

Math object

Math（ Count ） The object's function is ： Perform ordinary arithmetic tasks .

Math Object provides a variety of numeric types and functions . There is no need to define this object before using it .

Use Math Properties of / The grammar of the method ：

example

var x=Math.PI;
var y=Math.sqrt(16);

Be careful ： Math Object does not need to be defined before using this object .

Numerical value

JavaScript Provide 8 Species can be Math Object access arithmetic value ：

You can refer to the following Javascript Constant usage ：

example

Math.E
Math.PI
Math.SQRT2
Math.SQRT1_2
Math.LN2
Math.LN10
Math.LOG2E
Math.LOG10E

Arithmetic method

In addition to being Math Object to access the arithmetic value , There are also several functions （ Method ） have access to .

The following example uses Math Object's round Method to round a number .

document.write(Math.round(4.7));

The above code output is ：

The following example uses Math Object's random() Method to return a value between 0 and 1 Random number between ：

document.write(Math.random());

The above code output is ：

0.06654728055558401

The following example uses Math Object's floor() Methods and random() To return a value between 0 and 11 Random number between ：

document.write(Math.floor(Math.random()*11));

The above code output is ：

JavaScript Array（ Array ） object

The function of an array object is ： Use a separate variable name to store a series of values .

Create array , For the assignment ：

example

var mycars = new Array();
mycars[0] = "Saab";
mycars[1] = "Volvo";
mycars[2] = "BMW";

Try it »

You can find more examples at the bottom of the page .

What is an array ?

Array objects use separate variable names to store a series of values .

If you have a set of data （ for example ： Car name ）, There are separate variables as follows ：

var car1="Saab";
var car2="Volvo";
var car3="BMW";

However , If you want to find a car ？ And not 3 car , It is 300 What about cars? ？ It will not be easy ！

The best way is to use arrays .

An array can store all values with a variable name , And you can access any value with a variable name .

Each element in the array has its own ID, So that it can be easily accessed .

Create an array

Create an array , There are three ways .

The following code defines a myCars Array object of ：

1: The conventional way :

var myCars=new Array();
myCars[0]="Saab";      
myCars[1]="Volvo";
myCars[2]="BMW";

2: Concise way :

var myCars=new Array("Saab","Volvo","BMW");

3: Literally :

var myCars=["Saab","Volvo","BMW"];

Access array

By specifying the array name and index number , You can access a particular element .

The following examples can be accessed myCars The first value of the array ：

var name=myCars[0];

The following example modifies the array myCars The first element of :

myCars[0]="Opel";

You can have different objects in an array

be-all JavaScript Variables are objects . Array elements are objects . Functions are objects .

therefore , You can have different variable types in the array .

You can include object elements in an array 、 function 、 Array ：

myArray[0]=Date.now;
myArray[1]=myFunction;
myArray[2]=myCars;

Array methods and properties

Use array objects to predefine properties and methods ：

var x=myCars.length             // myCars  Number of elements in 
var y=myCars.indexOf("Volvo")   // "Volvo"  The index value of the value

Create new methods

The prototype is JavaScript Global constructors . It can build new Javascript Properties and methods of objects .

example ： Create a new method .

Array.prototype.myUcase=function(){
    for (i=0;i<this.length;i++){
        this[i]=this[i].toUpperCase();
    }
}

Try it »

The above example creates a new array method to convert lowercase characters of the array to uppercase characters .

This is the end of today's sharing , Friends passing by have money to hold a money market , Pay attention if you have no money , Bloggers will be closed when they see the meeting. If you read this article, you can review it. If you haven't read it, you can learn a new knowledge. Thank you .

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