当前位置：网站首页>Flow control statement of avascript

Flow control statement of avascript

2022-07-24 07:10:00 【Mushroom poison】

JavaScript Process control statement of

1. if Judgment statement
2. if...else sentence
3. switch Select statement
4. while sentence
5. do...while sentence
6. for sentence

1. if Judgment statement

if Statement is the most commonly used conditional selection statement , Every programming language has one or more forms if sentence ;
if The syntax of the statement is as follows ：

if( Conditional statements ){
    
	 Execute statement ;
	}

Conditional statements It can be any kind of logical expression , if Conditional statements The return result of is TRUE, Then the program executes first followed by braces { } Right Execute statement , Then execute other subsequent statements ;
if Conditional statements The return result of is FALSE, Then the program code skips Conditional statements , Directly execute other statements after the program code ;
Curly braces { } The function of is to combine multiple statements into a compound statement , Deal with as a whole , If there is only one statement in braces , Braces can be omitted { };

Example 1：

<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
	</head>
	<body>
		<script> var b = 5; if(b>4) alert("b>4"); </script>
	</body>
</html>

20220530 if The statement will first judge b Is the value greater than 4, If the condition holds , eject “b>4” A hint of , Otherwise nothing will be carried out ;

2. if…else sentence

if…else Statement is usually used when a condition requires two program branches to execute ;
The syntax format is as follows ：

if( Conditional statements )
{
	 Execute statement  1;
}
else
{
	 Execute statement  2;
}

stay if Add a else clause , In this way, when the return result of the conditional statement is FALSE when , perform else The code after the clause ;

Example 2：

<html>
	<head>
		<meta charset="utf-8">
		<title></title>
	</head>
	<body>	
	<script> var a =12; if(a==10) {
       alert("a==10"); } else {
       alert("a!=10"); } </script>
	</body>
</html>

20220530

3. switch Select statement

switch Selection statements are often used to compare the result of an expression with multiple values , And select the execution statement according to the comparison result ;
The syntax format is as follows ：

switch( expression )
{
	case  Value  1 :
		 sentence  1;
		break;
	case  Value  2 :
		 sentence  2;
		break;
	...
	case  Value  n :
		 sentence  n;
		break;
		default :
			 Sentence block  n+1;
}

Example 3：

<html>
	<head>
		<meta charset="utf-8">
		<title>switch  Application of selection statement </title>
		<script type="text/javascript"> function btnOK(){
       var txtBookId = document.all.txtBookId.value; txtBookId = parseInt(txtBookId); switch(txtBookId) {
       case 1: window.alert(" Your chosen major is ： Civil engineering ！"); break case 2: window.alert(" Your chosen major is ： Computer ！"); break case 3: window.alert(" Your chosen major is ： Preschool education ！"); break case 4: window.alert(" Your chosen major is ： Marketing Management ！"); break default: window.alert(" Please enter the correct serial number ！") document.all.txtBookId.focus(); } } </script>
	</head>
	<body>
		<form action="#" name="frmSwitch">
			<table align="center" width=100% border="1" cellspacing="0">
				<tr>
					<td colspan="2"> Enter the group purchase commodity number ：</td>
				</tr>
				<tr>
					<td>1： Civil engineering </td>
					<td>2： Computer </td>
				</tr>
				<tr>
					<td>3： Preschool education </td>
					<td>4： Marketing Management </td>
				</tr>
				<tr>
					<td colspan="2">
						<input type="text" name="txtBookId">&nbsp;&nbsp;&nbsp;&nbsp;
						<input type="button" value=" choice " onclick="btnOK()">
					</td>
				</tr>
			</table>
		</form>
	</body>

20220530

4. while sentence

while Statement is a circular statement , It is also a judgment sentence ;
The syntax format is as follows ：

while( Conditional expression statements )
{
	 Execute statement ;
}

When Conditional expression statements The return value of is TRUE when , Then execute braces { } The statement in , When the braces are executed { } After the statement in , Check the conditional expression again , If the return value is TRUE, Then repeat the brace { } The statement in , Know that the return value is FALSE, End the cycle , And then go on while The program code after the statement segment ;

Example 4：

<html>
	<head>
		<meta charset="utf-8">
		<title></title>
	</head>
	<body>
		<table width=50% border="1">
			<script> var n=1; while (n<=8) // loop 8 Time  {
       "<tr>" document.write("<td align=center>"+" The first  "+n+"  Column </td>"); n++; } "</tr>" </script>
		</table>
	</body>
</html>

20220530

5. do…while sentence

do…while The function of the statement is similar to while Statement rice , However, it only detects after the first cycle Conditional expression Value , That is, the statement contained in braces must be executed at least once ;
do…while At the end of the statement while There is a semicolon after the parenthesis of the conditional statement “;”;
The syntax format is as follows ：

do
{
	 Execute statement 
}
while( Conditional expression statement );

Example 5：

<html>
	<head>
		<meta charset="utf-8">
		<title></title>
	</head>
	<body>
		<script> var m=1; do {
       document.write("<p> This is the first "+m+" individual "); m++; }while(m<=8); </script>
	</body>
</html>

20220530

6. for sentence

for The statement consists of two parts , One part is the condition control part , Part is the circulation part ;
The syntax is as follows ：

for( Initialization expression ; Loop condition expression ; Operation expression after loop )
{
	 Execute statement ;
}

Use for Set a counter number before cycling , Can be found in for Set before loop , It can also be defined in use ;
Initialization expression Indicates the initial value of the counter variable ;
Loop condition expression Is an expression of a counter variable , Determines the maximum value of the counter ;
The expression of Cao group after the loop Represents the step size of the loop , That is, not once , Change of counter quantity value , The change can be increased , Or reduced , Or other operations ;
for Loops can be nested ;

Example 6：

<html>
	<head>
		<meta charset="utf-8">
		<title>for  Statement example </title>
	</head>
	<body>
		<script> for(var m=9;m>=1;m--) {
       for (var n=9;n>=m;n--) {
       if(n*m<10) {
       document.write("&nbsp;"); } document.write(n*m+"&nbsp;"); } document.write("<br>") } </script>
	</body>
</html>