Javascript Statement（上）
目录一、JS基础知识
1. js引用
<!--Mathod 1-->
<head>
<script src="js_path"></script>
</head>
<!--Mathod 2-->
<body>
<script>
statement
</script>
</body>
2. Javascript构成
- ECMASript: 核心部分，包含js的语法
- DOM: 文件对象模型 (整合HTML, CSS, JS)
- BOM: 浏览器模型 (整合JS和浏览器)
3. 定义变量
如果不用var声明，则该变量默认为全局变量
var a=1;
var a=1, b=2;
var s="hello",f=3.2;
4. 变量命名规则
- 只能以字母、下划线、$美元符开头，且区分大小写
- Camel标记法：首字母小写，其他单词以大写字母开头
- Pascal标记法：每个单词的首字母都大写
- 匈牙利标记法：在Pascal标记的前提下，首字母需为小写的数字类型(如：iMyFirstArg; sMySecondArg)
5. 注释方式
- 单行注释：//
- 多行注释：/*content*/
6. 常量和标识符
自定义标识符: sayHello
字符串常量: "nihao"
保留字: function; var
<script>
function sayHello(){
var sHelloFirstArg="nihao"
document.write(sHelloFirstArg)
}
sayHello();
</script>
二、JS数据类型
1. 基本数据类型
2. 数据类型的转换
字符串拼接引起的自动转换
- 数字+字符串→字符串
- 数字+布尔值→数字（true→0；false→1）
- 布尔值+字符串→字符串（true→"true"；false→"false"）
转换函数
NaN: 当字符串转换数字失效时，会得到NaN这个值，归属于Number类型。但NaN是一个空值，不会和任何数据包括它自己相等
parseInt(); //将数据转换成整型
typeof(); // 返回数据类型
三、JS运算符
1. 算数运算符
// 基本运算符（+ - * / ++ --）
var a = 1;
var b = a++ // b=1
var b = ++a // b=2
2. 一元运算符
If "b" is the string of digital type, it will be automatically turned back the "b" of digital type but will get a NAN value.
var a = +b
var a = -b
3. 逻辑运算符
"&&" and "!!" can get a value not belonging the boolen type, if the result on one side is not the type of boolen.
// && !! !
2>1 && 3>5 // false(Equate "and" in python)
2>1 !! 3>5 // true(Equate "or" in python)
2 ! [3,5] // true(Equate "not" in python)
4. 位运算符
&: only when the two number are 1, the counting result equate 1,but 0 in binary system's counting
|: the result of same number is itself. but 1 in binary system's counting
^: the result of same number is 0. but 1 in binary system's counting
// all numbers shift two digits westwards, and the empty position will be filled by zero in the binary system
<<
// all numbers shift two digits eastwards, and the empty position will be filled by zero in the binary system
>>
// all zero and one exchange position for all numbers
~
// 位与(&)
1 & 2 = 0
/*
0 1
1 0
*/
// 位或(|)
1 | 2 = 3
/*
0 1
1 0
*/
// 异或(^)
1 ^ 2 = 3
/*
0 1
1 0
*/
5. 等性运算符
notice
null == undefined return ture
before checked if two characters are equation, all counting numbers tend to become the type of Number,having a priority of switch.
// one of the two characters Will change type if the two characters are the type that can be switched by each other,then return true or false
==
!=
// one of the two characters Will not change type, then return true or false
===
!==
6. 关系运算
// > < >= <=
alert(25<3) // false
alert("25"<"3") // true (Each characters base on the position on the ASCI to judge, if the comparation have not the result)
alert("25"<3) // false
7. 布尔值为false的值
flase;
[];
0;
null;
undefined;
四、循环判断语句
1. if语句
if (condition){
statement;
}else if(condition){
statement;
}else{
statement;
}
2.switch选择控制语句
var x = 3;
switch(x){
case 1:statement; break;
case 2:statement; break;
case 3:statement; break;
default:statement;
}
3. for 语句
// Method 1
var a=[1,"hello",true];
for (var i in a){
console.log(i);
console.log(a[i]);
}
// Method 2
for (var i=1; i<10; i=i+1){
console.log(i)
}
4. while 循环
var sum = 0;
var flag = 1;
while(flag<101){
sum += flag;
++flag;
}
5. 异常处理
throw Error("xxx") (Artificially thorw the information of error)
try{
statement;
}
catch(e){
statement;
}
finally{
statement;
}