# 流程控制
# 分支控制
# if语句
判断语句的语法格式为：
if(判断条件){
执行语句
}
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来看下面实例：
<html>
<head></head>
<body>
<script type="text/javascript">
var val = 20;
if (val > 90) {
console.log("优秀")
} else if (val > 60) {
console.log("及格")
}
var obj1 = []
var obj2 = {}
var obj3 = [1]
var str1;
var str2 = "";
var str3 = "2"
judgeObj(obj1)
judgeObj(obj2)
judgeObj(obj3)
judgeObj(str1)
judgeObj(str2)
judgeObj(str3)
function judgeObj(obj) {
function IfNull(obj) {
console.log("判断对象不会undefined，null，且不为空字符串，对象里面有属性。")
if (obj != null && obj.length > 0) {
console.log("对象不为空");
}
}
function IfNull2(obj) {
console.log("判断对象不会为undefined，null，且字符串不为空。")
if (obj) {
console.log("对象不为空2")
}
}
IfNull(obj);
console.log("------")
IfNull2(obj)
console.log("----------------")
}
</script>
</body>
</html>
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# switch语句
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>switch语句</title>
<script>
var a;
switch (a) {
case 1:
alert("number");
break;
case "abc":
alert("string");
break;
case true:
alert("true");
break;
case null:
alert("null");
break;
case undefined:
alert("undefined");
break;
}
</script>
</head>
<body>
</body>
</html>
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# 循环控制
# for语句
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>for语句</title>
<script>
var sum = 0;
for (var i = 1; i <= 100; i ++){
sum += i;
}
alert(sum);
</script>
</head>
<body>
</body>
</html>
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# for/in语句
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>for in语句</title>
<script>
var person = { name: "xie", age: 22 };
var x;
for (x in person) {
console.log(x + ":" + person[x]);
}
</script>
</head>
<body>
</body>
</html
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# forEach语句
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>forEach语句</title>
<script>
var temp=[3,2,9,16]
temp.forEach(function(val,index,array){
console.log(val+":"+index+":"+array[index])
})
</script>
</head>
<body>
</body>
</html>
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# while语句
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>while语句</title>
<script>
var sum = 0;
var num = 1;
while(num <= 100){
sum += num;
num ++;
}
console.log(sum);
var sum2 = 0;
do{
sum2+=num;
num --;
}while(num>=0);
console.log(sum2);
</script>
</head>
<body>
</body>
</html>
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# 实例
写一个9*9乘法表：
<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>99乘法表</title>
<style>
td{
border: 1px solid;
}
</style>
<script>
document.write("<table align='center'>");
for (var i = 1; i <= 9 ; i++) {
document.write("<tr>");
for (var j = 1; j <=i ; j++) {
document.write("<td>");
document.write(i + " * " + j + " = " + ( i*j) +" ");
document.write("</td>");
}
document.write("</tr>");
}
document.write("</table>");
</script>
</head>
<body>
</body>
</html>
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