Isolated Modules option

By default ts-jest uses TypeScript compiler in the context of a project (yours), with full type-checking and features. But it can also be used to compile each file separately, what TypeScript calls an 'isolated module'. That's what the isolatedModules option (which defaults to false) does.

You'll lose type-checking ability and some features such as const enum, but in the case you plan on using Jest with the cache disabled (jest --no-cache), your tests will then run much faster.

Here is how to disable type-checking and compile each file as an isolated module:

Example

JavaScript

// jest.config.js
/** @type {import('ts-jest').JestConfigWithTsJest} */
module.exports = {
  // [...]
  transform: {
    // '^.+\\.[tj]sx?$' to process ts,js,tsx,jsx with `ts-jest`
    // '^.+\\.m?[tj]sx?$' to process ts,js,tsx,jsx,mts,mjs,mtsx,mjsx with `ts-jest`
    '^.+\\.tsx?$': [
      'ts-jest',
      {
        isolatedModules: true,
      },
    ],
  },
}

TypeScript

// jest.config.ts
import type { JestConfigWithTsJest } from 'ts-jest'

const jestConfig: JestConfigWithTsJest = {
  // [...]
  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',
      {
        isolatedModules: true,
      },
    ],
  },
}

export default jestConfig

JSON

// package.json
{
  // [...]
  "jest": {
    "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",
        {
          "isolatedModules": true
        }
      ]
    }
  }
}

Performance

Using isolatedModules: false comes with a cost of performance comparing to isolatedModules: 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.