Error Handling in JavaScript: Try, Catch, Finally

While writing JavaScript programs, errors are unavoidable. Sometimes a variable is undefined, sometimes user input is invalid, and sometimes something unexpected happens during execution.

If errors are not handled properly, the program may stop running completely.

That’s why JavaScript provides error handling tools like:

• try

• catch

• finally

These help programs fail gracefully instead of crashing suddenly.

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What Are Errors in JavaScript?

An error is a problem that occurs while the program is running.

Example:

console.log(username);

Output:

ReferenceError: username is not defined

Here, JavaScript throws an error because the variable does not exist.

Some common JavaScript errors are:

Error Type	Meaning
ReferenceError	Variable does not exist
SyntaxError	Invalid syntax
TypeError	Invalid operation on a value

Errors are useful because they help developers identify problems in code.

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Why Error Handling Matters

Without error handling, a single error can stop the execution of the entire program.

Good error handling helps:

• prevent application crashes

• improve debugging

• display user-friendly messages

• make applications more reliable

Instead of showing confusing errors to users, developers can handle issues properly and continue execution safely.

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Understanding Graceful Failure

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Using try and catch

The try block contains code that may produce an error.

The catch block runs if an error occurs.

Syntax

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

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Example

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

Output:

Something went wrong

Instead of crashing, the program handles the error safely.

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Understanding the Error Object

The catch block receives an error object that contains information about the problem.

Example:

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

Output:

userName is not defined

This is useful for debugging.

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Error Handling Flow

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The finally Block

The finally block always executes whether an error occurs or not.

It is commonly used for cleanup tasks.

Example

try {
  console.log("Inside try block");
} catch(error) {
  console.log("Error occurred");
} finally {
  console.log("Finally block executed");
}

Output:

Inside try block
Finally block executed

Even if there is no error, the finally block still runs.

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Try → Catch → Finally Execution Order

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Throwing Custom Errors

JavaScript also allows developers to create their own errors using the throw keyword.

Example:

function checkAge(age) {
  if (age < 18) {
    throw new Error("Age must be 18 or above");
  }

  console.log("Access granted");
}

try {
  checkAge(15);
} catch(error) {
  console.log(error.message);
}

Output:

Age must be 18 or above

Custom errors help make validation and debugging clearer.

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Real-World Use Cases

Error handling is used in many practical situations:

• validating form input

• handling API request failures

• checking login credentials

• reading files

• handling payment processing errors

Without proper error handling, applications would be much harder to use and debug.

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Assignment Practice

Create a simple try-catch example.

try {
  console.log(user);
} catch(error) {
  console.log("Variable not found");
}

Use the finally block.

try {
  console.log("Trying...");
} catch(error) {
  console.log("Error");
} finally {
  console.log("Execution completed");
}

Throw a custom error.

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);
}

Experiment with different errors in the browser console to better understand how JavaScript handles unexpected situations.