JavaScript if else and else if (conditional statements)

Conditional statements are used to perform different actions based on different conditions.

Conditional Statements

Very often when you write code, you want to perform different actions for different decisions.

You can use conditional statements in your code to do this.

In JavaScript we have the following conditional statements:

• Use if to specify a block of code to be executed, if a specified condition is true
• Use else to specify a block of code to be executed, if the same condition is false
• Use else if to specify a new condition to test, if the first condition is false
• Use switch to specify many alternative blocks of code to be executed

Note: The switch statement is described in the next chapter.

The if Statement

Use the if statement to specify a block of JavaScript code to be executed if a condition is true.

Syntax

                      if (condition) {
                        //  block of code to be executed if the condition is true
                      }
                

Note: Note that if is in lowercase letters. Uppercase letters (If or IF) will generate a JavaScript error.

Example

Make a "Good day" greeting if the hour is less than 18:00:

                      if (hour < 18) {
                        greeting = "Good day";
                      }
                

The else Statement

Use the else statement to specify a block of code to be executed if the condition is false.

                      if (condition) {
                        //  block of code to be executed if the condition is true
                      } else { 
                        //  block of code to be executed if the condition is false
                      }
                  

Example

If the hour is less than 18, create a "Good day" greeting, otherwise "Good evening":

                    if (hour < 18) {
                      greeting = "Good day";
                    } else {
                      greeting = "Good evening";
                    }
                

The else if Statement

Use the else if statement to specify a new condition if the first condition is false.

Syntax

                      if (condition1) {
                        //  block of code to be executed if condition1 is true
                      } else if (condition2) {
                        //  block of code to be executed if the condition1 is false and condition2 is true
                      } else {
                        //  block of code to be executed if the condition1 is false and condition2 is false
                      }
                

Example

If time is less than 10:00, create a "Good morning" greeting, if not, but time is less than 20:00, create a "Good day" greeting, otherwise a "Good evening":

                      if (time < 10) {
                        greeting = "Good morning";
                      } else if (time < 20) {
                        greeting = "Good day";
                      } else {
                        greeting = "Good evening";
                      }
                

Exercises - test your knowledge

Exercise 1

x = 10

y = 5

Create a simple if statement to alert "Hello world" if x is greater than y.

                        x = 10
                        y = 5
                        
                        if (x > y) {
                          alert('Hello world');
                        }
      

Exercise 2

x = 10

y = 50

Create a simple if else statement to alert "Hello world" if x is greater than y and otherwise alert "Goodbye".

                        x = 10
                        y = 50
                        
                        if (x > y) {
                          alert("Hello world");
                        } else {
                          alert("Goodbye");
                        }