transpilation option

By default ts-jest uses TypeScript Compiler API aka full Program in the context of a project (yours), with full type-checking and features. But it can also be used to compile each file separately, which is usually named as transpilation like other tools Babel, swc, esbuild etc. That's what the transpilation option (which defaults to false) does.

You'll lose type-checking ability and all TypeScript limitations are applied in trading off for faster test running.

Here is the example how to use the option

Example

jest.config.ts

import type { Config } from 'jest'

const jestConfig: Config = {
  // [...]
  transform: {
    // '^.+\\.[tj]sx?$' to process js/ts with `ts-jest`
    // '^.+\\.m?[tj]sx?$' to process js/ts/mjs/mts with `ts-jest`
    '^.+\\.tsx?$': [
      'ts-jest',
      {
        transpilation: true,
      },
    ],
  },
}

export default jestConfig

Performance

Using transpilation: false comes with a cost of performance comparing to transpilation: true. There is a way to improve the performance when using this mode by changing the value of include in tsconfig which is used by ts-jest. The least amount of files which are provided in include, the more performance the test run can gain.

Example

tsconfig.json

{
  // ...other configs
  "include": ["my-typings/*", "my-global-modules/*"]
}

Caveats

Limiting the amount of files loaded via include can greatly boost performance when running tests. However, the trade off is ts-jest might not recognize all files which are intended to use with jest. One can run into issues with custom typings, global modules, etc...

The suggested solution is what is needed for the test environment should be captured by glob patterns in include, to gain both performance boost and avoid breaking behaviors.