JavaScript Exception Handling

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Exception handling in JavaScript is used to manage runtime errors without stopping the entire program. It helps developers write stable and reliable applications.

What is an Exception?

An exception is an unexpected error that occurs while the program is running.

Examples:

  • Dividing by invalid values
  • Accessing undefined variables
  • Invalid JSON parsing
  • Network/API failures
  • Custom validation errors

Why Exception Handling is Important

Exception handling helps to:

  • Prevent application crashes
  • Show meaningful error messages
  • Debug applications easily
  • Handle API/database failures gracefully
  • Improve user experience

JavaScript Error Types

1. Syntax Error

Occurs when JavaScript code has invalid syntax.

console.log("Hello"

Output:

SyntaxError: missing ) after argument list

2. Reference Error

Occurs when accessing an undefined variable.

console.log(userName);

Output:

ReferenceError: userName is not defined

3. Type Error

Occurs when an operation is performed on the wrong data type.

let num = 10;
num.toUpperCase();

Output:

TypeError: num.toUpperCase is not a function

4. Range Error

Occurs when a value is outside the allowed range.

let arr = new Array(-1);

Output:

RangeError: Invalid array length

5. URI Error

Occurs when URI functions are used incorrectly.

decodeURIComponent("%");

Output:

URIError: URI malformed

try…catch Statement

The try...catch block is the core of exception handling.

Syntax

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

Basic Example

try {
    let result = 10 / 0;
    console.log(result);
} catch(error) {
    console.log("Something went wrong");
}

Example with Actual Error

try {
    console.log(user);
} catch(error) {
    console.log("Error Name:", error.name);
    console.log("Error Message:", error.message);
}

Output:

Error Name: ReferenceError
Error Message: user is not defined

Error Object Properties

The error object contains useful information.

PropertyDescription
nameError type
messageError description
stackStack trace

finally Block

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

Syntax

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

Example

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

Output:

Start
Error handled
Execution completed

throw Statement

The throw keyword is used to create custom exceptions.

Syntax

throw "Custom Error";

or

throw new Error("Invalid input");

Custom Error Example

let age = 15;

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

    console.log("Eligible");
} catch(error) {
    console.log(error.message);
}

Output:

Age must be 18 or above

Nested try…catch

You can use multiple try-catch blocks.

try {
    try {
        console.log(userName);
    } catch(error) {
        console.log("Inner Catch");
    }

    console.log(data);
} catch(error) {
    console.log("Outer Catch");
}

Exception Handling in Functions

function divide(a, b) {
    try {
        if(b === 0) {
            throw new Error("Division by zero not allowed");
        }

        return a / b;

    } catch(error) {
        return error.message;
    }
}

console.log(divide(10, 0));

Exception Handling with JSON

Very common in API automation and backend development.

let jsonData = '{"name":"Deepesh"}';

try {
    let user = JSON.parse(jsonData);
    console.log(user.name);

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

Invalid JSON Example

let jsonData = '{"name":"Deepesh"';

try {
    let user = JSON.parse(jsonData);

} catch(error) {
    console.log("JSON Parsing Failed");
}

Async Exception Handling

Used with async/await.

Example

async function fetchData() {

    try {
        let response = await fetch("https://dummyjson.com/users/1");

        let data = await response.json();

        console.log(data);

    } catch(error) {
        console.log("API Error:", error.message);
    }
}

fetchData();

Best Practices

1. Use Specific Error Messages

throw new Error("Password cannot be empty");

2. Avoid Empty catch Blocks

❌ Bad

catch(error) {

}

✅ Good

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

3. Use finally for Cleanup

Useful for:

  • Closing database connections
  • Clearing resources
  • Stopping loaders

4. Do Not Overuse try…catch

Use it only around risky code.

Real-World Example

Form Validation

function login(username, password) {

    try {

        if(!username) {
            throw new Error("Username is required");
        }

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

        console.log("Login Successful");

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

login("", "12345");

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