How to handle errors in JavaScript?

What are errors?

Errors, in programming, occur when a program encounters obstacles that hinder its normal operation. Such situations arise when the program encounters challenges, such as attempting to access a file that does not exist or connecting to a web-based API endpoint without an available network connection. In these scenarios, the program encounters errors that prompt it to notify the user of its inability to proceed. The program diligently gathers relevant error information and communicates its inability to proceed any further.

JavaScript errors manifest as objects that are displayed whenever a programming error occurs. These error objects contain abundant information, including details about the error type, the specific statement that triggered the error, and the stack trace capturing the error's origin. JavaScript further empowers programmers to generate custom errors, enabling them to enhance error debugging by incorporating additional relevant information.

How to Identify and Prevent Errors in JavaScript?

1. Using try-catch blocks:

JavaScript provides a way to handle errors gracefully and prevent them from breaking the code. This is done by using the try...catch...finally statement, which allows you to define a block of code to be tested for errors, a block of code to handle any error that occurs, and a block of code to run regardless of the result.

The purpose of the try block is to enclose the code that has the potential to generate an error. If an error does occur within the try block, the catch block is responsible for executing the code specifically designed to handle that error. The catch block takes an error object, typically named "err" or another specified identifier, which holds relevant information about the error, including its name and message.

The throw statement allows you to create a custom error and pass it to the catch block. You can throw any value, such as a string, a number, a boolean, or an object.

On the other hand, the finally block is an optional block of code that follows the try-catch block. The code within the finally block is executed regardless of whether an error occurred or not. It provides a mechanism to ensure certain actions are taken, such as closing resources or releasing memory, regardless of the error outcome. For example, you can use it to validate user input and display a friendly message if the input is invalid:

function validateInput(input) {
  try {
    if (input.trim() == "") throw "empty";
    if (isNaN(input)) throw "not a number";
    input = Number(input);
    if (input < 0) throw "negative";
    if (input > 100) throw "too large";
    // code to process valid input
  } catch (err) {
    console.log("Invalid input: " + err);
  } finally {
    console.log("Thank you for using our service.");
  }
}

validateInput("50"); // valid input
// Thank you for using our service.

validateInput("abc"); // invalid input
// Invalid input: not a number
// Thank you for using our service.

By implementing the try-catch block, we can handle the error in a graceful manner and provide informative feedback to the user or developer. This approach prevents the error from propagating unchecked and potentially causing the program to crash.

1. Utilizing Error Objects:

In certain cases, you can utilize the name and message properties of the Error object to obtain a more specific message based on the error type. The Error object has its own subclasses such as TypeError, ReferenceError, SyntaxError, etc. The Error object has two properties: name and message, which can be used to identify and describe the error. The ‘name’ property indicates the general error class, such as DOMException or Error, while the ‘message’ property typically provides a concise message rather than converting the entire error object to a string.

When throwing custom exceptions, if you want to leverage these properties (especially if your catch block does not differentiate between your own exceptions and system-defined ones), you can utilize the Error constructor. This allows you to customize and enhance the error handling experience. The following code snippet throws an Error upon division by zero:

function divide(x, y) {
  try {
    if (y == 0) throw new Error("Division by zero");
    return x / y;
  } catch (err) {
    console.log(err.name + ": " + err.message);
  }
}

divide(10, 2); // returns 5
divide(10, 0); // throws an error
// Error: Division by zero

Conclusion

Like any other programming language, errors are quite frequent and natural in JavaScript. In some cases, you might even need to throw errors intentionally to indicate the correct response to your users. Hence, understanding their anatomy and types is very crucial.

Moreover, you need to be equipped with the right tools and techniques to identify and prevent errors from taking down your application.

In most cases, a solid strategy to handle errors with careful execution is enough for all types of JavaScript applications.