// writing

Slash Your ts-jest Test Times With This

1 min read

Our CI test suite took 10+ minutes. The ts-jest type-check was the bottleneck. One flag fixed it.

jest.config.js
module.exports = {
  preset: "ts-jest",
  globals: {
    "ts-jest": { isolatedModules: true },
  },
};

isolatedModules makes ts-jest compile each file on its own and skip the slow project-wide type analysis. The bigger the codebase, the bigger the win. Newer ts-jest reads it straight from your tsconfig.json.

The catch: Jest stops type-checking your code. Files compiled in isolation cannot catch errors that span files.

So make type-checking its own CI step:

Terminal window
tsc --noEmit

Fast tests, types still checked, and two jobs that stopped fighting each other.