Conditional Judgment Statement if statement/switch statement

• An if (conditional expression) statement

The IF statement is a conditional judgment statement, and for an expression in parentheses in the IF statement, ECMAScript automatically calls the Boolean () transformation function to convert the result of the expression to a Boolean value. If the value is true, execute the following statement, or do not execute the

 <script type= "Text/javascript" >var  box = 100; if  (Box > 50) // IF statement if it returns FALSE, only the following statement is not executed      alert (box);  //  The second statement is independent of the IF statement, so  alert is executed (" Whether it is true or FALSE, it will execute to!    ");  </script> 
 <script type= "Text/javascript" >var  box = 10< Span style= "color: #000000;" >; if  (Box > 50) {//  here is a compound statement treated as a statement, also called a code block   alert (box); Alert (, whether true or false, will execute to!)    "); }  </script> 

• if (conditional expression) {statement;} else{statement;}

<script type= "Text/javascript" >var box = +; if (Box >) {    alert(' box is greater than 'else  {    alert (' box less than ');} </script>

• if (conditional expression) {statement;} else if (conditional expression) {statement;} ... else{statement;}

<script type= "Text/javascript" >varbox = 98;if(Box >= 90) {alert (Methyl);} Else if(Box >= 80) {alert (Ethyl);} Else if(Box >= 70) {alert (Isopropyl);} Else if(Box >= 60) {alert (Failed);} Else{alert (' Fail ');} </script>

• Switch statement

Multiple conditional judgments, for comparison of multiple values equal

<script type= "Text/javascript" >varbox=2;Switch(box) {//The Switch box box is the variable you want to compare     Case1://Case 1: equivalent to the IF statement (box = = 1), if box is 1Alert (' One ');  Break;//Break exits in the middle to prevent penetration     Case2://if (box = = 2)Alert (' Both ');  Break;  Case3: Alert (' Three ');  Break; default://equivalent to the else in the IF statementAlert (' Error! ‘);}</script>

Conditional Judgment Statement if statement/switch statement