A three introduction method
1, the introduction of internal
<script>
alert ( "aaa"); // written in the head tag
</ script>
<script type="text/javascript">
alert ( "aaa"); // write in the body tag
</ script>
The best writing in a recent body from the inside
2, the line is introduced
<p onclink="javascript:alert('hell word');">clickme</p>
3, the introduction of external
HTML： <script src="js/style.js"></script>
JS： document.write("hell word!")
Second, variable
var variable name = variable value;
var to declare variables, variable name can contain letters, numbers, underscores and must begin with a letter or an underscore, strictly case-sensitive name, FirstName and firstName is different.
// a statement to declare variables
There are a = 2, b = 4, c = 6;
// declare variables can span multiple lines
There are a = 2,
b = 4,
c = 6;
<script>
There are a = 3;
a = 5; // variables can be declared without var Again
document.write (a); // 5, the same name will have to cover the variable name
</script>
// hump labeling and underlined Act
var firstName = 'king'; // small hump
var FirstName = 'qween'; // large hump
var first-name = 'ymt'; // underlined Method
Two types of variables case
1, alert (a); // when unquoted variable is not defined, the browser will complain, a is not defined a not defined
2, var a; // when no assignment of a defined, no error will be prompted undefined undefined
alert(a);
Third, the output
. 1, <Script>
Alert ( "AAA"); // Alert warning window pops
</ script>
2、<script>
document.write ( "hello word!") ; // in HTML can display output content, write labels
document.write("<h1>标题一</h1>");
</script>
. 3, <Script>
the console.log ( "console"); // displayed on the console, F12 ----> select Console
</ Script>
Fourth, the conversion
1, replaced by other types of false must have Boolean type
var a; //undefined ----> false
var a=""; //""------->false
null == undefined
"" == 0
2, other types of conversion into numerical
var b=undefined; //undefined ------> NaN
b= null; //null--------->0
b =true; // true ---------->1
b = flase; //flase----------->0
b = '12'; //'12' ---------->12
b = '3king'; //'3king'--------->NaN
NaN: number is not a number, can not be compared with any number, including itself.
Fifth, operators
1, arithmetic operators
+ Addition
- Subtraction
* Multiplication
/ Division
% Remainder
++ increment, a ++ output and first increment, ++ a first output increase since
- Self-cut, with a ++
2, join operator
+ May be summed string
3, the assignment operator
=
4, comparison operators
== equal, compare values
=== congruent, only compare values further comparison data type
! = Not equal
! The whole is not equal ==
> Greater than
<Less than
> = Greater than or equal to
<= Less than or equal to
5, logical operators
&& is a logical AND flase if the first, the second operation is not skipped, the equivalent of breaking
|| or when the first logic is true, the second operation is not skipped, the equivalent of a short circuit
! Logical NOT
6, the ternary operator
if(3>1){
"aa"
}else{
"bb"
}
var res = 3> 1 "aa": "bb"; // this is a ternary operator, when 3> 1 is true, take?? Behind the 'aa'; when 3> 1 is false, take? Behind the 'bb'.
7, comma operator
A plurality of operation may be performed in a single statement by comma operator
Six other
<noscript>
Your browser does not support JavaScript, please see the replacement. // can identify the <script> tag where the browser does not support script
</noscript>