javascript条件语句(JavaScript常用的输出语句有哪些)

What are JavaScript Conditional Statements?

JavaScript Conditional statements are used to execute specific blocks of code if certain conditions are met. Similar to decision-making in everyday life, these statements make decisions in the program as well. The program will execute specific code only if certain conditions are met. In essence, conditional statements are used to create a more dynamic and responsive program that can respond to different situations and conditions.

Types of Conditional Statements in JavaScript

There are several types of conditional statements in JavaScript. The most common are the if statement, the if-else statement, the else-if statement, and the switch statement. The basic syntax of an if statement is:

   if (condition) {
       // Code to be executed if the condition is true
   }

The if-else statement can be used to execute different code depending on whether the condition is true or false. The basic syntax of an if-else statement is:

   if (condition) {
       // Code to be executed if the condition is true
   } else {
       // Code to be executed if the condition is false
   }

The else-if statement is similar to the if-else statement, but can be used to check multiple conditions. The basic syntax of an else-if statement is:

   if (condition1) {
       // Code to be executed if condition1 is true
   } else if (condition2) {
       // Code to be executed if condition2 is true
   } else {
       // Code to be executed if none of the conditions are true
   }

The switch statement is used to compare a value against multiple possible cases and execute the code associated with the first matching case. The basic syntax of a switch statement is:

   switch (expression) {
       case value1:
           // Code to be executed if expression matches value1
           break;
       case value2:
           // Code to be executed if expression matches value2
           break;
       default:
           // Code to be executed if expression does not match any of the cases
           break;
   }

Examples of Conditional Statements in JavaScript

Let's take a look at some examples of conditional statements in JavaScript. Here is an example of an if statement:

   let age = 25;

   if (age >= 18) {
       console.log("You are an adult.");
   }

This code will check if the value of age is greater than or equal to 18. If it is, the code inside the if statement will be executed and the message "You are an adult." will be printed to the console.

Here is an example of an if-else statement:

   let num = 10;

   if (num % 2 == 0) {
       console.log("The number is even.");
   } else {
       console.log("The number is odd.");
   }

This code will check if the value of num is even or odd. If it is even, the message "The number is even." will be printed to the console. If it is odd, the message "The number is odd." will be printed to the console.

Here is an example of an else-if statement:

   let grade = "B";

   if (grade == 'A') {
       console.log("You got an A!");
   } else if (grade == 'B') {
       console.log("You got a B!");
   } else if (grade == 'C') {
       console.log("You got a C.");
   } else {
       console.log("You failed the class.");
   }

This code will check the value of grade and print a message depending on the grade. If the grade is 'A', it will print "You got an A!". If the grade is 'B', it will print "You got a B!". If the grade is 'C', it will print "You got a C.". If the grade is anything else, it will print "You failed the class.".

Conditional statements are a vital part of programming and can help create robust and responsive programs. By using conditional statements, programmers can create applications that are capable of making decisions and responding to different situations.