Slash Your ts-jest Test Times With This

Okay, so our CI pipeline was starting to feel like a coffee break extending into lunch – TypeScript tests were the main culprit. After some digging, I stumbled upon a ridiculously simple one-liner for ts-jest that clawed back over 10 minutes for us. Figured I’d share the wealth!

Here’s the little gem:

jest.config.js
module.exports = {
  preset: 'ts-jest',
  globals: {
    'ts-jest': {
      isolatedModules: true,
    }
  },
  // ... rest of your config
};

How this works

Basically, isolatedModules: true tells ts-jest to process each TypeScript file as a completely separate unit. It skips the full project-wide type analysis that normally happens. Think of it like a quick, focused compilation for each file in isolation, rather than tsc looking at the whole picture. This is what gives you that massive speed boost, especially noticeable with larger codebases.

⚠️ The Crucial Caveat: Don’t Skip Your Type Checks!

This speed-up comes with a big string attached: Jest will no longer be doing type-checking for you.

Since isolatedModules treats files individually, it can’t catch type errors that depend on understanding your whole project. So, it’s absolutely critical that you have a separate, dedicated step in your CI pipeline to handle type verification (e.g., running tsc --noEmit or vue-tsc --noEmit for Vue projects). If you skip this, you’re essentially flying blind on type safety during your tests, and you will risk introducing type-related bugs into your system.

Pro Tip / Future-Proofing: While tweaking globals works, the ts-jest folks and general TypeScript wisdom now point towards sticking isolatedModules: true right into your tsconfig.json (under compilerOptions).