Technical point 3: Javascript language

Irving the procedural ape 2020-11-09 01:10:04
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 the literal values :=== 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/java.........
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