9.Error_Handling.md

100 JS Questions (Error Handling)

Q1. What is Error Handling in JS?

Error handling is the process of managing errors

// try block contains the code that might throw an error
try {
  const result = someUndefinedVariable + 10; // This line will throw an error because someUndefinedVariable is not defined
  console.log(result);
} catch (error) {
  // catch block is where the error is handled
  console.log('An error occurred:', error.message);
}

// Output:
// An error occurred: someUndefinedVariable is not defined

Q2. What is the role of finally block in JS?

Finally, block is used to execute some code irrespective of error.

• Without Exception

  If there is no exception then everything runs normally under try then moves to finally.

• With Exception

  If there is an exception. Then try run till error, then catch runs and then finally runs

try {
  const result = someUndefinedVariable + 10;
  console.log(result);
} catch (error) {
  console.log('An error occurred:', error.message);
} finally {
  console.log("finally executed");
}

Q3. What is the purpose of the throw statement in JS? V. IMP.

The throw statement stops the execution of the current function and passes the error to the catch block of the calling function.

• Method 1 (try-catch)
  • Method 2 (throw)

function UserData() {
  try {
    validateUserAge(25);
    validateUserAge("invalid"); // This will throw
    validateUserAge(15); // This will not execute
  } catch (error) {
    console.error("Error:", error.message);
  }
}

function validateUserAge(age) {
  if (typeof age !== "number") {
    throw new Error("Age must be a number");
  }
  console.log("User age is valid");
}

Q4. What is Error propagation in JS?

Error propagation refers to the process of passing or propagating an error from one part of the code to another by using the throw statement with try-catch.

• Method 1 (try-catch)
  • Method 2 (throw)

function UserData() {
  try {
    validateUserAge(25);
    validateUserAge("invalid"); // This will throw
    validateUserAge(15); // This will not execute
  } catch (error) {
    console.error("Error:", error.message);
  }
}

function validateUserAge(age) {
  if (typeof age !== "number") {
    throw new Error("Age must be a number");
  }
  console.log("User age is valid");
}

Q5. What are the best practices for error handling?

// 1. Use Try Catch and Handle Errors Appropriately
try {
  // Code that may throw an error
} catch (error) {
  // Error handling and recovery actions
}

// 2. Use Descriptive Error Messages
throw new Error("Cannot divide by zero");

// 3. Avoid Swallowing Errors
try {
  // Code that may throw an error
} catch (error) {
  // Do not leave the catch blank
}

// 4. Log Errors
try {
  // Code that may throw an error
} catch (error) {
  console.error("An error occurred:", error);
  // Log the error with a logging library
}

Q6. What are the different types of errors In JS?

Error Type	Description	Example
SyntaxError	An error in the syntax of the JavaScript code.	console.log("Hello, World!); (Missing closing parenthesis)
ReferenceError	An error that occurs when a variable or object is accessed before it is declared or defined.	console.log(myVariable); (myVariable is not defined)
TypeError	An error that occurs when an operation is attempted on an object of an incorrect type.	const number = 42; console.log(number.toUpperCase()); (number.toUpperCase is not a function)
RangeError	An error that occurs when a numeric value falls outside of the allowed range for a specific operation.	const array = [1, 2, 3]; console.log(array[10]); (Index 10 is out of bounds)

// Syntax Error
console.log("Hello, World!");
// Missing closing parenthesis );

// Reference Error
console.log(myVariable);
// myVariable is not defined

// Type Error
const number = 42;
console.log(number.toUpperCase());
// number.toUpperCase is not a function

// Range Error
const array = [1, 2, 3];
console.log(array[10]);
// Index 10 is out of bounds