First, figure out what kinds of errors might pop up in the system. These could be server errors, client errors, validation errors, or network issues. Knowing the common error types will help you create solid error handling mechanisms.

Handle errors right in the client-side code using JavaScript. Use try...catch blocks around code that might throw errors. This way, you can catch errors as they happen and deal with them smoothly.

try {
    // Code that might throw an error
} catch (error) {
    console.error("An error occurred:", error);
    // Display a user-friendly message
    alert("Something went wrong. Please try again later.");
}

Create and use custom error messages that are specific to your app, so users and developers know exactly what went wrong. You can do this by creating a custom Error class.

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

Make sure all user inputs are validated before processing them. This prevents many types of errors related to invalid data. Use JavaScript libraries or HTML5 validation attributes to handle input validation.

<input type="email" required>

if (!email.includes("@")) {
    throw new CustomError("Invalid email address.");
}

Send error details to a server for logging and analysis. This helps in monitoring and fixing issues that might not be immediately visible from the user interface.

function logErrorToServer(error) {
    fetch("/log-error", {
        method: "POST",
        body: JSON.stringify({ error: error.message, stack: error.stack }),
        headers: {
            "Content-Type": "application/json"
        }
    });
}

try {
    // Code that might throw an error
} catch (error) {
    logErrorToServer(error);
}

Make sure that error messages shown to users are user-friendly and don't expose internal error details. This is important for security and user experience.

try {
    // Code that might throw an error
} catch (error) {
    const userMessage = "An unexpected error occurred. Please try again later.";
    alert(userMessage);
    logErrorToServer(error);  // Log the detailed error for developers but show a generic message to users
}

Set up global error handlers to catch errors that aren't caught by specific error handling code. In a web app, this can be done using window.onerror and window.onunhandledrejection.

window.onerror = function (message, source, lineno, colno, error) {
    logErrorToServer(error);
    alert("A global error occurred. Please try again later.");
};

window.onunhandledrejection = function (event) {
    logErrorToServer(event.reason);
    alert("A promise error occurred. Please try again later.");
};

Thoroughly test all error handling mechanisms to make sure they work as expected. Write unit tests for error-prone functions and simulate various error conditions to verify the robustness of error handling.

// Example test using Jest
test('should handle error gracefully', () => {
    function failingFunction() {
        throw new Error("Test error");
    }
    expect(() => failingFunction()).toThrow("Test error");
});