Java script basic concepts, variables, constants

1 , JavaScript Concept

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

2. 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

3. Basic grammar

* 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:/* */

4. 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]

5. 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

Java script basic concepts, variables, constants