getElementByTagName

return an array with element with tag name, with order they appear in the page

ex: 

document.getElementByTagName("div")

innerHTML

the content of tag element

usage:  get or change the content of element

ex:

document.getElementById("test").innerHTML=  "change";

regular expression in javascript

syntax:

/  expression  /

. : match any character other than newline

\s:  match a whitespace character

\d:  match number

\w: match alphanumeric character 

^:  beginning of the string

$:  end of the string

\: escape character

quantifiers:

* :  o or more times

+:  1 or more times

?:  o or 1 time

{ n} :  n times

{min, max}:   

[abc]:  a or b or c

regular expression is a RegExp object

how to use:

ex:

var  key= /\d/;

if( key.test( "32") )

{

     alert("matched");

}

control logic ( if & switch)

switch , case, break , default:
the same as C syntax

if , else :
the same as C syntax

comment

single line: //

multiple lines: /* and */

prompt

let user enter data

ex:

prompt("what is your name",  "insert name here");

import javascript into html

<script type="text/javascript"  src="test.js"> </script>