JS的条件与循环语句简介

条件语句

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If语句

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if语句的基本样式

if ("条件判断语句") {

...

}

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if-else语句的基本样式

if ("条件判断语句") {

...

}else {

...

}

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If语句的嵌套

<script>

var score = prompt("请输入您的期末考试成绩");

var daily = prompt("请输入您的考勤成绩");

if (score > 60&&daily > 75){

alert("及格")

}else {

if (score < 60&&daily <75){

alert("成绩和考勤不及格")

}else if (score < 60){

alert("成绩不及格")

}else if(daily <75){

alert("考勤不及格")

}

}

</script>

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Switch语句

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Switch的基本结构

Switch ("必须是常量"){

case1("常量"):

...;

break;

case2("常量"):

...;

break;

.

.

.

}

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Switch语句的用法

<script>

var month = parseInt(prompt("请输入月份"));

switch (month){

case 1:

case 2:

case 3:

alert("第一季度");

break;

case 4:

case 5:

case 6:

alert("第二季度");

break;

case 7:

case 8:

case 9:

alert("第三季度");

break;

case 10:

case 11:

case 12:

alert("第四季度");

break;

default:

alert("输入的月份有误");

}

</script>

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If语句和Swicth语句的套用

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<script>

var day = parseInt(prompt("请输入一个0~6的数"));

if (day >= 1&&day <= 5){

switch (day){

case 1:

alert("星期一 工作日");

break;

case 2:

alert("星期二");

break;

case 3:

alert("星期三");

break;

case 4:

alert("星期四");

break;

case 5:

alert("星期五");

break;

}

alert("工作日")

}else if (day == 0||6){

switch (day){

case 0:

alert("星期天");

break;

case 6:

alert("星期六");

break;

}

alert("非工作日")

}

</script>

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循环语句

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js的循环语句类型：while

do while

for

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循环结构必备三要素：

1.循环变量初始化的定义

2.循环变量的判断

3.循环变量的更新

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while循环语句的基本结构

while（“判断语句”）{

循环体语句；

}

<script>

var sum = 0;

var num = 1; ....................循环变量的定义

while (num <= 10){ ......................循环变量的判断

if (num % 2 == 0){

sum = sum + num;

num++; .........................循环变量的更新

}else {

num++

}

}

alert(sum);

</script>

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do while循环语句的基本结构

do {

循环体语句；

}while (“判断语句”）;

<script>

var sum = 0;

var num = 1; ....................循环变量的初始化

do{

sum += num;

num++;...........................循环变量的更新

}

while (num <= 10); .....................循环变量的判断

alert(sum);

</script>

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for循环语句的基本结构

for(初始化循环变量;循环条件;修改循环变量的值）

{

循环体语句

}

<script>

var i = 1;

var sum = 0;

for (i = 1;i <=10;i++){

sum = sum + Math.pow(i,2);

}

alert(sum);

</script>