astTransformers.md

title
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

/** @type {import('ts-jest').JestConfigWithTsJest} */
module.exports = {
  // [...]
  transform: {
    '<regex_match_files>': [
      'ts-jest',
      {
        astTransformers: {
          before: ['my-custom-transformer'],
        },
      },
    ],
  },
}

import type { JestConfigWithTsJest } from 'ts-jest'

const jestConfig: JestConfigWithTsJest = {
  // [...]
  transform: {
    '<regex_match_files>': [
      'ts-jest',
      {
        astTransformers: {
          before: ['my-custom-transformer'],
        },
      },
    ],
  },
}

export default jestConfig

{
  // [...]
  "jest": {
    "transform": {
      "<regex_match_files>": [
        "ts-jest",
        {
          "astTransformers": {
            "before": ["my-custom-transformer"]
          }
        }
      ]
    }
  }
}

Configuring transformers with options

/** @type {import('ts-jest').JestConfigWithTsJest} */
module.exports = {
  // [...]
  transform: {
    '<regex_match_files>': [
      'ts-jest',
      {
        astTransformers: {
          before: [
            {
              path: 'my-custom-transformer-that-needs-extra-opts',
              options: {}, // extra options to pass to transformers here
            },
          ],
        },
      },
    ],
  },
}

import type { JestConfigWithTsJest } from 'ts-jest'

const jestConfig: JestConfigWithTsJest = {
  // [...]
  transform: {
    '<regex_match_files>': [
      'ts-jest',
      {
        astTransformers: {
          before: [
            {
              path: 'my-custom-transformer-that-needs-extra-opts',
              options: {}, // extra options to pass to transformers here
            },
          ],
        },
      },
    ],
  },
}

export default jestConfig

{
  // [...]
  "jest": {
    "transform": {
      "<regex_match_files>": [
        "ts-jest",
        {
          "astTransformers": {
            "before": [
              {
                "path": "my-custom-transformer-that-needs-extra-opts",
                "options": {} // extra options to pass to transformers here
              }
            ]
          }
        }
      ]
    }
  }
}

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.