JavaScript Cookbook, 3rd Edition

Chapter 10. Errors and Testing

To write code is to write errors. Often, an error can be anticipated. Risky activities include actions that interact with outside resources (like files, databases, or web server APIs). Information that comes from outside your code—whether you’re reading it from a web page form or receiving it from another library—may arrive with errors, or in a different form than you expect. But to modify a well-worn cliché, it’s not so much the error as what you do with it that matters.

What should we do with our errors, then? JavaScript’s default behavior is to die at the point of the error, quietly logging a stack trace to the console. However, better options are available. You can capture an error, react to it, modify it, rethrow it, and even hide it if you choose. Compared to many other languages, JavaScript’s error-handling features are relatively underdeveloped. But basic error handling is still just as important, and many of the recipes in this chapter focus on that task.

Defending against errors is essential practice, but it’s equally important to prevent them wherever possible. To that end, there are many testing frameworks that work with JavaScript, including Jest, Mocha, Jasmine, and Karma. With their help, you can write unit tests that guarantee your code is executing as expected. You’ll take a quick look at Jest in this chapter.

Catching and Neutralizing an Error

Problem

You are performing a task that may not succeed, and you don’t ...