Learn JavaScript notes from the beginning (i)--basics

Summary: The composition of JavaScript, the role of each component,

First, the composition of JavaScript

	Javascript
ECMAScript (CORE)	DOM (Document Object model)	BOM (Browser object model)

1.1ECMAScript

ECMAScript is a standardized scripting language through ECMA-262, ECMA-262 language: syntax, type, statement, keyword, reserved word, operator, object

1.2 DOM

The DOM maps the entire page to a multi-tiered node structure, and each component of the page, such as HTML or XML, is a node of a certain type, and these nodes contain different types of data.

1.3 BOM

Controls the part of the browser that is displayed outside the page

Second, <script> elements

2.1 How to use

Externally referencing JavaScript files:

　　　　

<script type= "Text/javascript" src= ". /.. /xx.js "></script>

View Code

　Page embedding JavaScript code

<script type= "Text/javascript" >    var first= "First variable";    alert (firstly);</script>

2.2 Properties of the <script> element

Defer delay script: The script will be deferred until the entire page is parsed before execution, although it is deferred execution, but the browser has downloaded the JS file.

<! DOCTYPE html>
In the above example, although the <script> tag is within the 
Defer only applies to externally introduced script files
Async Async Script: Instead of having the page wait for the script to be downloaded and executed, the page can load other content asynchronously. Therefore, the asynchronous load script does not modify the DOM during the load.
<! DOCTYPE html>
Learn JavaScript notes from the beginning (i)--basics