What is a control statement in javascript

In JavaScript, a control statement is a statement structure used to control the execution order of each statement in the program. It can control the selection, looping, steering and return of the program flow. Control statements can be divided into three categories: sequence structures, selection structures and loop structures.

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

What is a control statement

A control statement is a statement structure used to control the execution order of each statement in the program. It can select, loop, turn and return the program flow. Wait for control.

Function: Used to control the flow of the program to realize various structural methods of the program.

Control statements can be divided into three categories:

• The first category is the sequential structure: from top to bottom, from left to right

• The second type is the selection structure: branch selection

• The third type is the loop structure: repeated execution

1. if control statement

##1. if...else:

if (表达式) {
语句１;
......
}
else {
语句２;
.....
}
//功能说明:
//如果表达式的值为true则执行语句1
//否则执行语句2

2 , if can be used alone: ​​

var x= (new Date()).getDay();//获取今天的星期值,0为星期天
var y;
if ((x==6)||(x==0)) {
y="周末";
}
else {
y="工作日";
}
alert(y);

//等价于:
y="工作日";
if ((x==6)||(x==0)) {
y="周末";
}

3, if nested:

if (表达式1) {语句1;}
else if (表达式2) {语句2;}
else if (表达式3) {语句3;}
else {语句4;}
//功能说明:
//如果表达式1的值为true则执行语句1
//如果表达式2的值为true则执行语句2...
//否则执行语句4

if (x==1){
    y="星期一";
}else if (x==2){
    y="星期二";
...
}else if (x==6){
    y="星期六";
}else if (x==0){
    y="星期日";
}else{
    y="未定义";
}

2. Switch selection control statement

1. Basic format:

switch (表达式) {
    case 值1:语句1;break;
    case 值2:语句2;break;
    case 值3:语句3;break;
    default:语句4;//其他均不成立时执行
}
//功能说明:
//如果表达式的值是值1/2...则分别执行语句1/2...然后退出
//如不为列出的任何值,执行default后的语句(语句4)
//不加break会继续执行下一个条件的语句
//default放于最后,或加break,否则执行下一条语句

var x=3
switch(x){
case 1:y="星期一";break;
case 2:y="星期二";break;
case 3:y="星期三";break;
case 4:y="星期四";break;
case 5:y="星期五";break;
case 6:y="星期六";break;
case 7:y="星期日";break;
default: y="未定义";
}
alert(y)
//case 3后不加break会继续执行y='星期四'...

2. The switch structure is more concise and clear than the if structure, making the program more readable and efficient

/*if语句适用范围比较广,只要是boolean表达式都可以用if判断
而switch只能对基本类型进行数值比较
两者的可比性就仅限在两个基本类型比较的范围内

/*说到基本类型的数值比较,那当然要有两个数
然后重点来了:*/
//if语句每一句都是独立的，看下面的语句:
if (a == 1) ...
else if (a == 2) ...
/*这样a要被读入寄存器两次:1和2分别被读入寄存器一次
其实a读两次是有点多余的,在全部比较完之前只需一次读入寄存器就即可,其余都是额外开销
但是if语句必须每次都把里面的两个数从内存拿出来读到寄存器,它不知道其实比较的是同一个a*/
//于是switch case就出来了,把上面的改成switch case版本:
switch (a) {
        case 0:
                break;
        case 1:
}
                
//总结:
1.switch用来根据一个整型值进行多路分支,并且编译器可以对多路分支进行优化
2.switch-case只将表达式计算一次,然后将表达式的值与每个case的值比较,进而选择执行哪一个case的语句块
3.if的判断条件范围较广,每条语句基本上独立的,每次判断时都要条件加载一次
所以在多路分支时用switch比if结构要效率高

3. While loop control statement

while (条件) {
语句1；
...
}
//功能说明:
//功能和for类似,当条件成立循环执行{}内语句,否则跳出循环

var i=1;
while (i<=7) {
    document.write("<H"+i+">hello</H "+i+"> ");
    document.write("<br>");
    i++;
}
//循环输出H1到H7的字体大小

<script language="JavaScript">
//sayhello是定义的函数名，前面必须加上function和空格
function sayHello(){
    var hellostr;
    var myname=prompt("请问您贵姓？","苑");
    hellostr="您好，"+myname+&#39;先生，欢迎进入"探索之旅"！&#39;;
    alert(hellostr);
    document.write(hellostr);
}
//对前面定义的函数进行调用
sayHello();
</script>

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