JavaScript Basics (Concepts, constants, and variables)

1 , JavaScript Concept

JavaScript is a scripting language; After writing, you can run it directly (the process of compiling is missing)

2 , JavaScript Development

LiveScript + JavaScript = Jscript = ECMAScript (standard for scripting languages)

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95 Netscape, Netscape United Sun, United Microsoft, tri-European Association of Computer Manufacturers, Inc.

Note: JavaScript syntax refers to the Java language; but "JavaScript is the scripting language of the Java language" is wrong

3 , JavaScript Component

ECMAScript: The standard of language (definition grammar) since the introduction of ES6 in 2015; A new version will be introduced every year in the future

Dom-document object Model (Document object models)

Bom-browser Object Model

4 , how to use JavaScript

Scripting JavaScript language with <script> elements:

(1) Define <script> elements, write directly

(2) through the <script> elements, the introduction of external JS file format: <script src= "js file path" ></script>

Type Property-Sets the currently introduced file types (text/javascript)

Note:<script> elements are written by default in

Even if it is written in an incorrect location: The browser does not have any errors, and the browser automatically resolves to change the error location to the correct location. (HTML is not a strict markup language)

5 , Test JavaScript Code

? Confirm (');

? alert ('); Rarely used at the moment (print content limited-not conforming to development requirements)

? prompt ('); Receive user information, the page has an input box to fill in the information

? document.write ('); The content of the test outputs the message directly on the page, and can be written with HTML tags.

? Console.log ('); Output messages in the console (console) in the browser's developer tool

Format: Console. Log(' log content '); Console. Warn(' output warning ');

Console. Info(' output information '); Console. Error(' output error ');

6 , basic syntax

* Case Sensitive Eg:atguigu, Atguigu and Atguigu-for three content

* Code of the normative

HTML: The element name is suggested using lowercase; Parent and child elements, recommended indentation (Space or TAB)

CSS: Combo selector #id,. class;

The specific wording of the CSS declaration eg: selector {

background-color:red;

}

* JavaScript: A line is best to write only one statement; It is recommended to write a semicolon after each statement;

Add a blank line between the two statements, and write a clear comment;

Keywords and reserved words cannot be used to define variable names;

Keywords: have special meanings in JavaScript code; Reserved words: Keywords that are not yet enabled

(Single-line comment) symbol://[shortcut: CTRL +?] (Multiline comment) symbol:/* */

7 , Variables

Concept: A container for storing data information. Format: var variable name = data content;

Features: Allows for repeated use of specific data functions: Allows the reuse of a specific data content

ü Declaration of variables

Duplicate declaration: A JavaScript variable can store only one data, and the previously stored data is overwritten.

Omission statement: Directly read the value of a non-declared variable, JavaScript will error.

Eg:console.log (test1); --Error because there is no definition test1

ü Naming rules

Start with a letter, underscore-, dollar sign $;   Does not begin with a number; Uppercase and lowercase letters do not conflict;

Do not use keywords and reserved words as names; names should have a clear meaning.

ü Usage

2 Defining variables-Define a variable name for a data

When there are multiple variables: (1) Each row defines a variable (2) Single row to define multiple variables, separated by commas

2 Call variable-use the data content of the variable

Define variables first, then call variables [normal]

When the browser loads the execution of the JavaScript code-from top to bottom, when a variable is called, the variable already exists

? Call the variable first, then define the variable---"Undefined"

Console.log (TEST2);    ==> var test2; --Define the variable (but not assigned value); The default value is "undefined"

var test2 = 300; Console.log (TEST2);

Test2 = 300; --Assigning values to variables

[Duplicate declaration: The corresponding data content will be overwritten with duplicate assignment: the corresponding data content will be overwritten]

8 , Constants

Format: const constant name = data content; Function: Represents a specific data content

Note: A variable that is read-only (cannot be modified), and the constant name is used in all capitals

* Constant declaration, must be initialized operation, otherwise it will report an error.

* Whether repeated or repeated assignment-constant will be an error

JavaScript Basics (Concepts, constants, and variables)