AST transformers option

ts-jest by default does hoisting for a few jest methods via a TypeScript AST transformer. One can also create custom TypeScript AST transformers and provide them to ts-jest to include into compilation process.

The option is astTransformers and it allows ones to specify which 3 types of TypeScript AST transformers to use with ts-jest:

• before means your transformers get run before TS ones, which means your transformers will get raw TS syntax instead of transpiled syntax (e.g import instead of require or define ).
• after means your transformers get run after TS ones, which gets transpiled syntax.
• afterDeclarations means your transformers get run during d.ts generation phase, allowing you to transform output type declarations.

Examples

Basic Transformers

jest.config.ts

import type { Config } from 'jest'

const jestConfig: Config = {
  // [...]
  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',
      {
        astTransformers: {
          before: ['my-custom-transformer'],
        },
      },
    ],
  },
}

export default jestConfig

Configuring transformers with options

important

The options config option will be serialized by jest-worker therefore only SERIALIZABLE values are allowed.

jest.config.ts

import type { Config } from 'jest'

const jestConfig: Config = {
  // [...]
  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',
      {
        astTransformers: {
          before: [
            {
              path: 'my-custom-transformer-that-needs-extra-opts',
              options: {}, // extra options to pass to transformers here
            },
          ],
        },
      },
    ],
  },
}

export default jestConfig

Writing custom TypeScript AST transformers

To write a custom TypeScript AST transformers, one can take a look at the one that ts-jest is using.