What Are Errors in JavaScript?

Errors occur when something goes wrong during code execution.

Examples:

• Invalid variable

• Wrong input

• Failed API request

• File not found

Runtime Error Example

console.log(userName);

Output

ReferenceError: userName is not defined

The program crashes because the variable does not exist.

Why Error Handling Matters

Error handling helps applications:

• Fail gracefully

• Prevent crashes

• Show meaningful messages

• Improve debugging

What Is try...catch?

try contains risky code.

catch handles errors if they occur.

Basic Syntax

try {
  // risky code
} catch (error) {
  // handle error
}

Example

try {
  console.log(userName);
} catch (error) {
  console.log("Something went wrong");
}

Output

Something went wrong

Instead of crashing, the error is handled safely.

Error Handling Flow

Accessing Error Information

try {
  console.log(userName);
} catch (error) {
  console.log(error.message);
}

Output

userName is not defined

The finally Block

finally always runs:

• Whether error happens or not

• After try and catch

Syntax

try {
  // code
} catch (error) {
  // handle error
} finally {
  // always runs
}

Example

try {
  console.log("Running code");
} catch (error) {
  console.log("Error occurred");
} finally {
  console.log("Cleanup completed");
}

Output

Running code
Cleanup completed

Example With Error

try {
  console.log(userName);
} catch (error) {
  console.log("Handled error");
} finally {
  console.log("Finally always runs");
}

Output

Handled error
Finally always runs

Try → Catch → Finally Flow

Throwing Custom Errors

You can manually create errors using:

throw

Example

function divide(a, b) {
  if (b === 0) {
    throw new Error("Cannot divide by zero");
  }

  return a / b;
}

try {
  console.log(divide(10, 0));
} catch (error) {
  console.log(error.message);
}

Output

Cannot divide by zero

Why Throw Custom Errors?

Custom errors help:

• Validate input

• Stop invalid operations

• Provide meaningful messages

Real-World Example

function login(password) {
  if (!password) {
    throw new Error("Password is required");
  }

  console.log("Login successful");
}

Common Error Types

Example: TypeError

const number = 10;

number.toUpperCase();

Output

TypeError

Numbers do not have toUpperCase().

Graceful Failure

Without error handling:

Application crashes

With error handling:

Application continues safely

Common Beginner Mistake

Incorrect:

try {
  console.log(userName);
}

Missing catch or finally.

Practical Example

try {
  const data = JSON.parse("{ invalid json }");

  console.log(data);
} catch (error) {
  console.log("Invalid JSON format");
}

Key Takeaways

Final Notes

Error handling is critical in real-world applications because failures always happen:

• Invalid user input

• Network issues

• Database failures

• API errors

Good error handling makes applications:

• Stable

• Easier to debug

• More user-friendly