You’re a TypeScript developer for a large company that sells stock photography products to the general public, as well as to news and other media markets. Your task is to find and fix some long-term bugs in a legacy library. You’ll write a test to characterize a bug in code that controls how certain assets are selected for a special display. After you’ve determined the source of the problem, you’ll modify the test, transforming it into a bug fix. Once you’ve squashed that bug, you’ll repeat the process to tackle two additional common bugs in TypeScript applications.

Your company sells stock photography products to the general public, as well as to news and other media markets. As the resident TypeScript developer, you’ve been tasked with maintaining and extending a class library for optimizing search results. You’ll set up tests to support refactoring of this legacy library, use “pinning tests” to extract a specification from the existing code, and generate a wide range of test coverage. You’ll learn to spot when and where to improve code maintainability, and set up mutation testing to ensure your tests catch unintended changes. By the end, you’ll have generated test coverage that supports refactoring, making future updates easy and painless.

The company you work for wants to improve its system by adding functionality that maximizes the profitability of any given search. As the resident TypeScript developer, your task is to apply a battery of tests to support changing the design of legacy code in preparation for adding this new functionality. You’ll refactor your production code based on advice from your architecture team. Thanks to a broad test suite, you’ll be able to change your design with no worries about accidentally breaking a required behavior. The result will be a system you can test with smaller, more meaningful tests and that easily withstands future requirement changes.

You're a TypeScript developer at an organization that sells stock photography products to the general public, as well as to news and other media markets. The company wants to add functionality to its system that maximizes the profitability of any given search. It’s up to you to improve the test suite by adding high-quality requirements-based tests. You’ll use mocks to isolate requirements, ensuring that each test focuses on a single behavior in your code. When you’re done, you’ll have a more resilient codebase.

As a TypeScript developer, you’ve recently created a high-quality test suite as part of your company’s goal of adding functionality to its system. Now, using test-driven development (TDD) techniques, it’s time to analyze new requirements, reconcile them with what is currently documented by your test suite, and use the test-first technique to ensure changes to your library are justified, specified, and protected by tests.