JavaScript Error and Exception Handling

When you write JavaScript code, sometimes things go wrong. These problems are called errors.

Understanding errors is very important because:

• They help you find mistakes in your code.
• They prevent your program from crashing unexpectedly.
• They make your applications more reliable.

What Is an Error?

An error is something that stops your program from running correctly.

In JavaScript, errors are generally grouped into:

1. Syntax Errors
2. Logical Errors
3. Runtime Errors (Exceptions)

Let’s explain each one clearly.

1️⃣ Syntax Errors

A syntax error happens when you break the rules of JavaScript grammar.

Think of it like a spelling or grammar mistake in English.

Example of a Syntax Error

console.log("Hello World"

❌ Problem: Missing closing parenthesis )

JavaScript will stop immediately and show an error like:

SyntaxError: missing ) after argument list

Another Example

let number = 10
if (number > 5 {
  console.log("Greater than 5");
}

❌ Missing closing parenthesis ) in if statement.

Important:

Syntax errors must be fixed before the program can run.

2️⃣ Logical Errors

A logical error happens when your program runs without crashing, but gives the wrong result.

This is harder to detect because there is no error message.

Example of Logical Error

let a = 10;
let b = 5;

let result = a - b;  // We meant to add

console.log(result);

If you wanted addition but wrote subtraction, the result will be wrong — but JavaScript will not complain.

Another Example

let age = 18;

if (age > 18) {
  console.log("You are an adult.");
}

Problem:

If age is exactly 18, nothing happens.

Maybe we meant:

if (age >= 18)

Important:

Logical errors are fixed by testing and debugging.

3️⃣ Runtime Errors (Exceptions)

A runtime error happens while the program is running.

These are also called exceptions.

Example: Trying to use something that doesn’t exist.

Example of Runtime Error

let user;
console.log(user.name);

❌ user is undefined, so accessing .name causes:

TypeError: Cannot read properties of undefined

Another Example

let number = 10;
number();  // Trying to call a number like a function

This causes:

TypeError: number is not a function

What Is Exception Handling?

Exception handling allows you to:

• Catch runtime errors
• Prevent program crashes
• Show friendly error messages

JavaScript uses:

• try
• catch
• finally
• throw

try...catch Syntax

try {
  // Code that might cause an error
} catch (error) {
  // Code that runs if an error occurs
}

Example

try {
  let user;
  console.log(user.name);
} catch (error) {
  console.log("An error occurred!");
}

Instead of crashing, it prints:

An error occurred!

Understanding the Error Object

Inside catch, the error object contains information.

try {
  let number = 10;
  number();
} catch (error) {
  console.log(error.name);     // TypeError
  console.log(error.message);  // number is not a function
}

finally Block

finally always runs — whether there is an error or not.

try {
  console.log("Start");
} catch (error) {
  console.log("Error happened");
} finally {
  console.log("This always runs");
}

throw Keyword

You can create your own errors using throw.

let age = 15;

try {
  if (age < 18) {
    throw "You must be 18 or older.";
  }
  console.log("Access granted");
} catch (error) {
  console.log(error);
}

Built-in JavaScript Error Types

Some common built-in errors:

Example of ReferenceError

console.log(x);  // x not declared

// 1️⃣ Logical Error Example
let a = 10;
let b = 5;

// We intended addition but used subtraction
let result = a - b;
console.log("Logical Error Result (should be 15):", result);


// 2️⃣ Runtime Error Example with try...catch
try {
  let user;
  console.log(user.name);  // This causes runtime error
} catch (error) {
  console.log("Caught an error!");
  console.log("Error Name:", error.name);
  console.log("Error Message:", error.message);
}


// 3️⃣ Using throw to create custom error
let age = 16;

try {
  if (age < 18) {
    throw new Error("You must be 18 or older.");
  }
  console.log("Access granted.");
} catch (error) {
  console.log("Custom Error:", error.message);
} finally {
  console.log("Age verification complete.");
}


// 4️⃣ Another Runtime Error Example
try {
  let number = 10;
  number();  // TypeError
} catch (error) {
  console.log("Another error caught:", error.message);
}