Technical point 3: Javascript language

Guo Shao 2020-11-09 00:08:09
technical point javascript language


JavaScript Introduction to language

One 、JavaScript Introduce

Javascript The birth of language is mainly to complete the data validation of the page . So it runs on the client side , You need to run a browser to parse and execute JavaScript Code .
JS yes Netscape Netscape products , It was first named LiveScript; To attract more java The programmer . Renamed JavaScript.
JS Weak type ,Java It's a strong type .

Two 、JavaScript and html The way code is combined

2.1、 The first way

Only need head In the label , Or in body In the label , Use script label To write JavaScript Code .

Code example :

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>script Use of labels </title>
<script type="text/javascript">
//alert yes JavaScript Language provides a warning box function 
// It can take any type of parameter , This parameter is the prompt message of the warning box 
 alert("hello javascript!");
</script>
</head>
<body>
</body>
</html>

The effect is as follows :

 

 

 

2.2、 The second way

Use script Tags introduced A separate JavaScript Code file .

Create a js file , Internal writing js Code :

 

 

 HTML The page content :

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>javascript</title>
<!--
We just need to pass src Attribute introduction js File can .
The way of introduction and directly in script The label says js There are only two ways to code , Can't be used at the same time
-->
<script type="text/javascript" src="1.js"></script>
<script type="text/javascript">
alert("hello2");
</script>
</head>
<body>
</body>
</html>

The effect is as follows :

 

 

3、 ... and 、 Variable

What is a variable ? A variable is a named memory that can hold certain values .

 

 

 

 

 

Code example :

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>javascript</title>
<script type="text/javascript">
var i;
// alert(i); //undefined
 i = 12;
// alert(typeof(i)); //number
// It can take the data type of any variable 
 i = "abc";
alert(typeof(i)); //string
var a = 12;
var b = "abc";
alert(a * b); //NaN
</script>
</head>
<body>
</body>
</html>

Four 、 Operator

4.1、 Relationship between operation

be equal to :==
It's a simple comparison of words
All equal to :===
In addition to making a literal comparison , It also compares the data types of the two variables

Code example :

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>javascript</title>
<script type="text/javascript">
var a = 12;
var b = "12";
alert(a == b);//true
 alert(a === b);//false
</script>
</head>
<body>
</body>
</html>

4.2、 Logical operations

&&: And operation

||: Or operations

!: Reverse operation

stay JavaScript In language , All variables can be used as boolean Type variables to use .

0、null、undefined、NaN、""( Empty string ) All are false.

 

 

Code example :

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>javascript</title>
<script type="text/javascript">
var a = "abc";
var b = true;
var c = false;
var d = null;
//&&
 alert(a && b);//true
 alert(b && a);//abc
 alert(a && c);//false
 alert(a && d);//null
//||
 alert(d || c);//false
 alert(c || d);//null
 alert(a || c);//abc
 alert(b || c);//true
</script>
</head>
<body>
</body>
</html>

5、 ... and 、 Array (***** a key )

5.1、 How to define an array

 

 

Code example :

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>javascript</title>
<script type="text/javascript">
var arr = [];
alert(arr.length);//0
 arr[0] = 12;
alert(arr[0]);//12
 alert(arr.length);//1
// stay JavaScript In language , We assign values to arrays by subscript , So the maximum subscript value , It will automatically expand the array 
 arr[2] = "abc";
alert(arr.length);//3
 alert(arr[1]);//undefined
// Traversal of array 
for (var i = 0; i < arr.length; i++) {
alert(arr[i]);
}
</script>
</head>
<body>
</body>
</html>

6、 ... and 、 function (***** a key )

6.1、 Two ways to define functions

The first one is : Use function Keyword to define the function

 Format :
function Function name ( Parameter list ){
The body of the function ;
}
// notes : If the function has a return value , Just use return Keyword return value .

Code example :

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>javascript</title>
<script type="text/javascript">
function fun1() {
alert(" Nonparametric functions fun1() It's out of use ");
}
fun1();// Function will only execute if it is called 
function fun2(a,b) {
alert(" There are parametric functions fun2() Is called the ,a = " + a + ", b = " + b);
}
fun2(10,20);
function sum(num1,num2) {
return num1 + num2;
}
alert(sum(10,20));
</script>
</head>
<body>
</body>
</html>

The second way to define it , The format is as follows :

var Function name = function( Parameter list ){
The body of the function ;
}

Code example :

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>javascript</title>
<script type="text/javascript">
var fun1 = function () {
alert(" Nonparametric functions ");
};
fun1();
var fun2 = function (a,b) {
alert("a = " + a + ", b = " + b);
};
fun2(10,20);
var sum = function (num1,num2) {
return num1 + num2;
};
alert(sum(10,20));
</script>
</head>
<body>
</body>
</html>

stay js in , Functions are not allowed to be overloaded , An overload of a function will override the last definition .

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>javascript</title>
<script type="text/javascript">
function fun() {
alert(" Nonparametric functions ");
}
function fun(a,b) {
alert(" There are parametric functions ");
}
fun();
</script>
</head>
<body>
</body>
</html>

The effect is as follows :

 

 

6.2、 Functional arguments Invisible parameter ( Only in function Within the function )

Is in the function There is no need to define , But it can be directly used to get all the parameters of the variable . We call it stealth parameters .

Code example :

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>javascript</title>
<script type="text/javascript">
function fun(a) {
alert("a = " + a);//a = 1
for (var i = 0; i < arguments.length; i++) {
alert(arguments[i]);
}
}
fun(1,true,"abc");
</script>
</head>
<body>
</body>
</html>

7、 ... and 、JS Custom objects in

 

 

Code example :

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>javascript</title>
<script type="text/javascript">
var obj = new Object();
obj.name = " Guo Peng ";
obj.age = 25;
obj.fun = function () {
alert(" Age :" + this.name + ", Age :" + this.age);
};
alert(obj.name);
obj.fun();
</script>
</head>
<body>
</body>
</html>

 

 

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>javascript</title>
<script type="text/javascript">
var obj = {
name:" Guo Peng ",
age:25,
fun:function () {
alert(" full name :" + this.name + ", Age :" + this.age);
}
};
alert(obj.age);
obj.fun();
</script>
</head>
<body>
</body>
</html>

8、 ... and 、js In the event

What is an event ?

The response of a computer input device to interact with a page , We call it an event .

 

 

Event registration can be divided into static registration and dynamic registration :

What is event registration ( binding )?
It's telling the browser , What operation code should be executed after the event response , It's called event registration or event binding .
Static registration Events : adopt html The event attribute of the tag is directly assigned to the code after the event response , This is called static registration .
Dynamic registration Events : To pass first js Code gets tagged dom object , And then through dom object . Event name = function(){} This form is given to the code after the event response , It's called dynamic registration .
Dynamic registration basic steps :
1、 Get the label object
2、 Label object . Event name = fucntion(){}

 

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本文为[Guo Shao]所创,转载请带上原文链接,感谢

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