2016.9.2博文分享！

运算符:

1.算术运算符( + - * / %)

加、减、乘、除、求余，其中%是求余运算

var total = (1 + 4) * 5;
var i = 100;
var temp = (i – 20) / 2;

alert(“10”+ 20) //return 1020;
alert(10 + 20) //return 30;

2. 后增量/后减量运算符 ++ ,--

var i = 10;
var a = i++; // i = i + 1;
alert(a);

例：

var i=20;
i+=1;
document.write(i+" ");
var i=20;
i+=10;
document.write(i+" ");
var i=20;
i-=1;
document.write(i+" ");
var i=20;
i-=10;
document.write(i+" ");

3. 比较运算符 ( >, <, >=, <=, ==, !=，===,!== )

alert ( 10 > 5 ); //outputs true

var i = 100;
var n = 100;
alert(i == n); //outputs true;
alert(i != n); //outputs false;
alert(i === n) //数据类型，值相等

例：

var price1=100,
price2=200;
document.write((price1 > price2)+" ");

var price1=200,
price2=100;
document.write((price1 < price2)+" ");

var price1=1199.98999999,
price2=1199.989999990000001;
document.write((price1 < price2)+" ");

var price1=999,
price2=1999;
document.write((price1 <= price2)+" ");

var number1=0,
string1="";
document.write((number1 == string1)+" ");

var number1=0,
number2=false;
document.write((number1 != number2)+" ");

var number1=0,
boolean1=false;
document.write((number1 === boolean1)+" ");

var boolean1=0,
boolean2=false;
document.write((boolean1 !== boolean2)+" ");

4.逻辑运算符( &&, || , !)

&& ：逻辑与
|| : 逻辑或
！：逻辑非

var i = 8;
alert ( i<5 && i<10); //outputs false

alert ( i > 100 || i < 10); //outputs true

alert(!(10 > 5)); //outputs false

例：

var num1=10,
num2=20;
document.write((num1>5 && num2<20)+" ");

var num1=33,
num2=34;
document.write((num1==33 && num2>20)+" ");

var boolean1=false,
boolean2=true;
document.write((boolean1 || !boolean2)+" ");
document.write((((((!boolean1 === boolean2) && !boolean2)|| !boolean1))===boolean2)+" ");

程序流程控制：

一、条件语句 if

语法：
if(condition) statements1 else statement2

例：
if(10 > 5) {
alert(“hello world”);
}

var i=60;
if(i>=80){
alert("成绩优异");
}else if(i<80 && i>=60) {
alert("成绩合格");
}else if(i<60){
alert("成绩不合格");
}

二、switch语句

语法：

switch (expression){
case value:
//statement
break;
case value:
//statement
break;
default:
//statement
}

例：

var i="A";
switch(i){
case 'A':
alert("你已经订购A套餐");
break;
case 'B':
alert("你已经订购B套餐");
break;
case 'C':
alert("你已经订购C套餐");
break;
case 'D':
alert("你已经订购D套餐");
break;
//默认
default:
alert("欢迎下次再来");
}