Summarize several simple Javascript judgment statements

Javascript is a popular scripting language used to add interactivity and dynamic effects to web pages. When writing Javascript programs, it is often necessary to make conditional judgments to perform different operations according to different situations. This article will introduce several simple Javascript judgment statements to achieve the required judgments when writing programs.

1. If statement

The if statement is one of the most commonly used conditional judgment statements. Its syntax structure is:

if(条件){
    //在条件为真时执行的代码
}

If the condition is true, the code in the curly brackets is executed. You can use comparison operators such as "==", "!=", ">", "=", and "

if(3>2){
    console.log("3大于2");
}

In the above example, the condition 3>2 is true, so "3 is greater than 2" will be output.

2. Else statement

The else statement is executed when the if condition is not established. Its syntax structure is:

if(条件){
    //当条件为 true 时执行的代码
}else{
    //当条件为 false 时执行的代码
}

If the condition is false, the code in the else statement is executed. For example:

var age = 18;
if(age>=18){
    console.log("成年人");
}else{
    console.log("未成年人");
}

In the above example, if age is greater than or equal to 18, then output "adult"; otherwise, output "minor".

3. Else if statement

The else if statement allows you to check another condition and execute the code when the if condition is not true. Its syntax structure is:

if(条件1){
    //当条件1为 true 时执行的代码
}else if(条件2){
    //当条件1为 false，条件2为true 时执行的代码
}else{
    //当条件1和条件2都为false时执行的代码
}

In the above syntax structure, you can add multiple else if statements. If condition 1 is false, condition 2 is checked and the code is executed if condition 2 is true. For example:

var score = 90;
if(score>=90){
    console.log("优秀");
}else if(score>=80){
    console.log("良好");
}else if(score>=60){
    console.log("及格");
}else{
    console.log("不及格");
}

In the above example, if the score is greater than or equal to 90, the output is "excellent"; if the score is greater than or equal to 80, but less than 90, the output is "good"; if the score is greater than or equal to 60, But if it is less than 80, then "pass" is output; otherwise, "fail" is output.

4. Switch statement

The switch statement is a statement used to execute code under multiple conditions. Its syntax structure is:

switch(表达式){
    case 值1:
        //当表达式等于值1时执行的代码
        break;
    case 值2:
        //当表达式等于值2时执行的代码
        break;
    ……
    default:
        //当表达式与所有值都不相等时执行的代码
        break;
}

In the above syntax structure, the expression can be any Javascript expression. If the expression equals a value, the code associated with that value is executed. If the expression is not equal to all values, the code in the default branch is executed. For example:

var day = new Date().getDay();
switch(day){
    case 0:
        console.log("星期日");
        break;
    case 1:
        console.log("星期一");
        break;
    case 2:
        console.log("星期二");
        break;
    ……
    default:
        console.log("不是星期几");
        break;
}

In the above example, the value of the day of the week is obtained through the Date object, and the code associated with it is executed in the switch statement. If day is equal to 0, then "Sunday" is output, if day is equal to 1, "Monday" is output, and so on. If day is not equal to any value from 0 to 6, output "not a day of the week".

Summary:

The above judgment statements are essential statements in implementing Javascript programs. When writing Javascript programs, choosing appropriate judgment statements according to different situations can greatly improve program efficiency and readability.

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