JavaScript gives HTML designers a programming tool,JavaScript can react to events,JavaScript can read and write HTML elements,JavaScript can be used to vilidate data,JavaScript can be used to detect the visitor's browser,JavaScript can be used to create cookies
The THML <script> tag is used to insert a JavaScript into an HTML page.
Browsers that do not support JavaScript,will display JavaScript as page content
JavaScript can be put in the <body> and in the <head> sections of an HTML page
execute <body> sections when page loading,and <head> sections will be executed when call it.
JavaScript can also be placed in external files
External JavaScript files often contain code to be uses on several different web pages.
External JavaScript files have the file extension .js
External script cannot contain the <script></script> tags
To use an external script,point to the .js file in the "src" attribute of the <script> tag.
create  cariables in JavaScript is most often referred to as "declaring" cairables,you declare JavaScript variables with the var keyboard
JavaScript Popup Boxes
JavaScript has three kind of popup boxes:Alert box,Confirm box,and Prompt box
Alet Box: An alert box is often used if you want to make sure information comes through to the user.When an alert box pops up,the user will have to click "OK" to proceed. alert("sometext");
The for...in statement loops through the properties of an object
By using Javascript,we have the ability to create dynamic web pages,Events are actions that can be detected by JavaScript.Every element on a web page has certain events which can trigger a Javascript.