kulshekhar/ts-jest

Jest is a popular tool for running automated tests in JavaScript projects. It works by executing your test files and checking that the code behaves the way you expect. TypeScript is a variant of JavaScript that adds type annotations, which help catch certain kinds of bugs before the code runs, but it needs to be translated into plain JavaScript before it can run in most environments. The problem ts-jest solves is that Jest does not understand TypeScript files out of the box. Without a bridge, you either have to pre-compile your TypeScript to JavaScript before running tests, or use a tool like Babel that strips the type information away without actually checking it. ts-jest is a plugin for Jest, called a transformer, that handles TypeScript files on the fly as Jest runs. It compiles each file, respects the TypeScript configuration of your project, and importantly, performs actual type-checking rather than just stripping the types. This means a type error in your code can cause a test to fail, giving you an extra layer of safety. Setting it up requires installing Jest and TypeScript first, then adding ts-jest alongside a type definitions package, and running a single command to generate the configuration file. After that, running your tests works the same way as any other Jest project. The version numbering for ts-jest follows Jest's own major version number rather than the standard semantic versioning convention. The project notes that version 23.10 is a significant rewrite, so projects on an older workflow may need to pin to an earlier version to avoid breaking changes. ts-jest is open source under the MIT license and is supported by JetBrains, which provides a free open-source license for the project's development tools.