JavaScript basics

• Introduction to JavaScript

Javascript has three important components, namely ECMAScript, DOM and BOM.

ECMAScript is the core of the language defined by the ECMA specification. In fact, the language itself does not contain input and output definitions. The ECMA specification defines only the basis of this language, and a more complete scripting language can be built on this basis. Our common web browser is just one of the possible host environments for ECMAScript implementation.

DOM, the Document Object Model, is an application programming interface for XML but extended for HTML. DOM maps the entire page into a multi-layer node structure. Each component in an HTML or XML page is a node of some type, and these nodes contain different types of data.

BOM, the Browser Object Model, provides a series of objects and APIs that can operate the browser.

• Apply javascript code

There are two ways to apply javascript code to the page.

Method 1: Insert javascript code into the <script> tag of the html document. </script>

Use script tags to wrap javascript code directly in the document. Some lower version browsers need to add the type="text/javascript" attribute to the script tag to correctly identify javascript code.

Method 2: Store the javascript code in a separate file and introduce the js file through the src attribute of the script tag.

Write the js code into a js file separately. When needed, introduce the js file through the script tag in the document.

• JavaScript syntax

1. Variables

Variables are defined by the var operator plus the variable name

Variable name naming rules: The first character must be a letter, underscore (_), or dollar sign ($); variable names cannot contain spaces or punctuation marks (except underscores (_) and $); variable names are case-sensitive ; Do not use reserved words or keywords. In order to make the variable name more readable, you can insert underscores appropriately in the variable name to separate them.

2. Data type

String

String type is used to represent a character sequence consisting of zero or more 16-bit Unicode characters, that is, a string. Strings can be represented by double
quotes (") or single quotes (')

Number

Number type

Uses IEEE754 format to represent
integers and floating-point values ​​(floating-point values ​​are also called double-precision values ​​in some languages).