About 01 Js & variable

Javascript Introduction

There are three front-end web:

HTML: from the semantic point of view, describe the structure of the page
CSS: From an aesthetic point of view, describing the style (beautification page)
JavaScript: From the point of view, describe the behavior of the interaction (enhanced user experience)

JavaScript composition

ECMAScript 5.0: js the standard defines the syntax: contains variables, expressions, operators, functions, IF statement for loop while loop, built-in functions
DOM: API operations on page elements. For example, let the box Show hidden, color, form validation animated form
BOM: API operate some functions of the browser. For example, refresh the page forward and back, let the browser automatically scroll as

Tool selection:

　　Common front-end development tools: sublime, visual Studio Code, HBuilder, Webstorm.

　　So PCharm with WebStorm you are using JetBrains Introduces editing tools, the development phase is recommended.

JS & reference variable naming:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-U-Compatible" content="IE-edge">
    <meta name="viewport" content="width=device-width,initial-scale=1">
    <title>01 Js简介&引入</title>
</head>
<body>
<!- Script<->inscribed type
    of the type = "text / JavaScript" > 
        // you write js code (two slashes // is a comment) 
        console.log ( ' the Hello World! ' ); // console output 
        Alert ( ' the Hello World! ' ) ; // browser box comes with warning shots 
    </ Script > 

<! - external -> 
<! - written into the src js file specified, similar to the python module in the introduction, the file specified the js are called here -> 
    < Script of the type = "text / JavaScript" src = "jQuery-3.3.1.js" > </ Script > 

<-! variables -> 
<! -Define the variable: var is a keyword used to define variables. The so-called keywords, that there are special features of small words. The keyword must be separated by a space. -> 
<! - assignment of variables: the equal sign represents the assignment, the value of the right-hand side, assigned to the variable on the left. -> 
<! - variable names: we can take to any variable name. -> 
    < Script type = "text / JavaScript" > 
        // directly define the variable var keyword assignment + declared variables js, note terminated with a semicolon a code 
        var STR = ' the Hello World! ' ; 
        the console.log (STR); 

        // assignment may define the 
        var A; 
        the console.log (A);   // console output undefined because this time is not defined to the variable 
        A =  10086 ; 
        the console.log (A); 

        //If there is no direct reference to the definition will complain 
        console.log (the X-);   //   console error: Uncaught ReferenceError: the X-IS not defined 
    </ Script > 

</ body > 
</ HTML >

Naming variables

　　There are naming the variable name: only by English letters, numbers, underscores, dollar sign $ constitution, and can not start with a number, JavaScript and can not be reserved words.