7. Conditional Statement and Exceptional Handling in Javascript

1. Conditional Statement

   1. if-else statement: The if-else statement executes a block of code if a specified condition evaluates to true, otherwise, it executes another block of code.

       let num = 10;
      
       if (num > 0) {
           console.log("The number is positive.");
       } else {
           console.log("The number is not positive.");
       }
      

2. if else-if else statement

    let marks = 75;

    if (marks >= 90) {
        console.log("Grade: A");
    } else if (marks >= 75) {
        console.log("Grade: B");
    } else {
        console.log("Grade: C");
    }

3. switch statement: The switch statement evaluates an expression and executes the corresponding case block based on the matching value.

let day = "Monday";

switch (day) {
    case "Monday":
        console.log("Start of the week!");
        break;
    case "Friday":
        console.log("Weekend is coming!");
        break;
    default:
        console.log("Regular day.");
}

2. Exception Handling

   JavaScript provides try, catch, finally, and throw to handle exceptions (errors) in a program.

   1. try-catch statement

      The try block contains the code that may throw an error, and the catch block handles the error.

       try {
           let x = y;  // y is not defined, this will cause an error
       } catch (error) {
           console.log("An error occurred:", error.message);
       }
      

2. finally statement

The finally block executes regardless of whether an error occurs or not.

    try {
        let num = 10;
        console.log(num.toUpperCase()); // Will cause an error
    } catch (error) {
        console.log("Error:", error.message);
    } finally {
        console.log("Execution completed.");
    }

    /* Output 
    Error: num.toUpperCase is not a function
    Execution completed.
    */

3. throw statement

The throw statement allows you to create custom errors.

function checkAge(age) {
    if (age < 18) {
        throw new Error("You must be 18 or older.");
    }
    return "Access granted!";
}

try {
    console.log(checkAge(16));
} catch (error) {
    console.log("Error:", error.message);
}
//output
//You must be 18 or older.

4. Error Objects

   JavaScript provides built-in error objects, such as:

   • Error: Generic error object.

   • ReferenceError: Occurs when a non-existent variable is accessed.

   • TypeError: Occurs when a value is not of the expected type.

   • SyntaxError: Occurs when there is a syntax mistake in the code.

       // ReferenceError Example
       try {
           console.log(x); // x is not defined
       } catch (error) {
           console.log("ReferenceError:", error.message);
       }
     
       // TypeError Example
       try {
           let num = 10;
           num(); // Trying to call a number as a function
       } catch (error) {
           console.log("TypeError:", error.message);
       }
     
       // SyntaxError Example (Uncommenting will cause script failure)
       // try {
       //     eval("console.log('Hello')"); // Invalid JavaScript code
       // } catch (error) {
       //     console.log("SyntaxError:", error.message);
       // }
     

INTERVIEW QUESTIONS

1. What is the difference between if-else and switch statements?

   • if-else is used for evaluating conditions with comparison operators (>, <, ===).

   • switch is used for checking multiple possible values of an expression.

2. What will be the output of following code ?

    let x = 5;
    if (x = 10) {
        console.log("x is 10");
    } else {
        console.log("x is not 10");
    }
   

   The output will be "x is 10" because x = 10 is an assignment, not a comparison (===), so the condition evaluates to true.

3. What will happen if there is an error inside the catch block?

   If an error occurs inside the catch block, it will not be handled by itself. You would need another try-catch to handle it.

4. What will be the output of following code ?

    try {
        console.log("Start");
        throw new Error("Something went wrong!");
        console.log("End"); // Will this execute?
    } catch (err) {
        console.log("Caught:", err.message);
    } finally {
        console.log("Finally executed");
    }
   

   Start

   Caught: Something went wrong!

   Finally executed

Next Topic :

8. Operators in JS