JavaScript Error Handling: Try..Catch..Finally, Throw & Custom Errors

In software engineering, runtime errors are inevitable—whether from invalid user input, network outages, or corrupted data payloads. Without robust exception handling, unhandled errors crash the JavaScript execution thread. The try..catch..finally statement constructs a resilient boundary that intercepts errors gracefully.

The Try..Catch..Finally Statement

try {
    // 1. Code that might throw a runtime exception
    console.log("Attempting database parse operation...");
    let result = JSON.parse("corrupted-raw-data-string");
    console.log("This line will never be reached!");
} catch (error) {
    // 2. Intercept and handle the error gracefully
    console.error("Error Caught:", error.name, "-", error.message);
} finally {
    // 3. ALWAYS executes unconditionally (Cleanup operations)
    console.log("Cleanup: Closing connection pool.");
}

Throwing Custom Exceptions & Custom Error Classes

You can throw native errors or create enterprise domain-specific error classes by extending the built-in Error class:

class ValidationError extends Error {
    constructor(message, field) {
        super(message);
        this.name = "ValidationError";
        this.field = field;
    }
}

function processPayment(amount) {
    if (typeof amount !== "number" || isNaN(amount)) {
        throw new TypeError("Payment amount must be a valid number.");
    }
    if (amount <= 0) {
        throw new ValidationError("Payment amount must be greater than zero.", "amount");
    }
    return `Processed payment of ₹${amount}`;
}

try {
    processPayment(-50);
} catch (err) {
    if (err instanceof ValidationError) {
        console.warn(`Validation failed on field [${err.field}]: ${err.message}`);
    } else {
        console.error("System Error:", err.message);
    }
}