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How to use if statement in javascript

青灯夜游Original: 2022-02-16 15:37:3616834browse

In JavaScript, the if statement is used to perform different operations based on different conditions. The syntax is "if (conditional expression) {// code to be executed;}" or "if (conditional expression) {//The code to be executed when the expression is true}else{//The code to be executed when the expression is not true}”.

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

In JavaScript, if statements are used to perform different operations based on different conditions.

if statements have three different forms:

if statement;
if else statement;
if else if else statement;

#if statement

if statement is the simplest conditional judgment statement in JavaScript. The syntax format is as follows:

if(条件表达式){
    // 要执行的代码;
}

When the conditional expression is established, that is, when the result is a Boolean value true, the code in { } will be executed.

The sample code is as follows:

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>JavaScript</title>
</head>
<body>
    <script type="text/javascript">
        var age = 20;
        if(age >= 18){      // 如果 age >= 18 的结果为 true，则执行下面 { } 中的代码
            alert("adult");
        }
    </script>
</body>
</html>

The running result is as shown below:

How to use if statement in javascript

if else statement

The if else statement is an upgraded version of the if statement. It can not only specify the code to be executed when the expression is true, but also the code to be executed when the expression is not true. The syntax format is as follows:

if(条件表达式){
    // 当表达式成立时要执行的代码
}else{
    // 当表达式不成立时要执行的代码
}

The sample code is as follows:

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>JavaScript</title>
</head>
<body>
    <script type="text/javascript">
        var now = new Date();           // 获取当前的完整日期
        var dayOfWeek = now.getDay();   // 获取一个 0-6 之间的数字，用来表示当前是星期几，0 表示星期日、1 表示星期一、以此类推
        if (dayOfWeek > 0 && dayOfWeek < 6) {       // 判断：如果当前是星期一到星期五中的一天，则输出“Have a nice day!”，若不是则输出“Have a nice weekend!”
            alert("Have a nice day!");
        } else {
            alert("Have a nice weekend!");
        }
    </script>
</body>
</html>

The running result is as shown below:

How to use if statement in javascript

##if else if else statement

If and if else statements both have only one conditional expression, and if else if else statement is their more advanced form, in if else if else statement allows you to define multiple conditional expressions and execute them based on the result of the expression The corresponding code, the syntax format is as follows:

if (条件表达式 1) {
    // 条件表达式 1 为真时执行的代码
} else if (条件表达式 2) {
    // 条件表达式 2 为真时执行的代码
}
...
  else if (条件表达式N) {
    // 条件表达式 N 为真时执行的代码
} else {
    // 所有条件表达式都为假时要执行的代码
}

Tip: During the execution of if else if else statement, when it encounters a established conditional expression, the code in { } will be executed immediately and then exit. The entire if else if else statement will not be executed if there is a valid conditional expression in the subsequent code.

The sample code is as follows:

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>JavaScript</title>
</head>
<body>
    <script type="text/javascript">
        var now = new Date();           // 获取当前的完整日期
        var dayOfWeek = now.getDay();   // 获取一个 0-6 之间的数字，用来表示当前是星期几，0 表示星期日、1 表示星期一、以此类推
        if(dayOfWeek == 0) {            // 判断当前是星期几
            alert("星期日")
        } else if(dayOfWeek == 1) {
            alert("星期一")
        } else if(dayOfWeek == 2) {
            alert("星期二")
        } else if(dayOfWeek == 3) {
            alert("星期三")
        } else if(dayOfWeek == 4) {
            alert("星期四")
        } else if(dayOfWeek == 5) {
            alert("星期五")
        } else {
            alert("星期六")
        }
    </script>
</body>
</html>

The running results are as shown below:

How to use if statement in javascript

Notes

When using nested if else, if there is only one line of statement, it should be wrapped in curly brackets to avoid condition ambiguity.

For example, the following nested if else can easily lead to misunderstanding:

if(0)
    if(1)
        console.log(1);
else
    console.log(0);

For the above code, the JavaScript interpreter will interpret it according to the following logical level based on the proximity principle:

if(0)
    if(1)
        console.log(1);
    else
        console.log(0);

So using braces can avoid many problems:

if(0){
    if(1) console.log(1);
}else{
    console.log(0);
}

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javascript learning tutorial]

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