How to Prevent TypeScript Type-Checking in Jest Tests?

If you wish to prevent TypeScript from type-checking your Jest tests, you can do either of the following:

1. Use babel-jest as a Transformer;
2. Use ts-jest as a Transform With isolatedModules.

#Using babel-jest as a Transformer

If you use babel-jest as a transformer to transpile TypeScript code into JavaScript, by default, it won't type-check your tests as they are run.

You can set it up in the following steps:

1. Install Relevant Packages

   npm install --save-dev babel-jest @babel/core @babel/preset-typescript @babel/preset-env
   

2. Specify Presets in Babel Configuration

   // babel.config.js
   module.exports = {
     presets: [
       ['@babel/preset-env', { targets: { node: 'current' } }],
       '@babel/preset-typescript',
     ],
   };
   

3. Set babel-jest as Transformer in Jest Configuration

   // jest.config.js
   module.exports = {
     // ...
     transform: {
       '^.+\\.tsx?$': 'babel-jest',
     },
     // ...
   }
   

#Using ts-jest as a Transform With isolatedModules

If you use ts-jest as a transformer to transpile TypeScript code into JavaScript, you can set it up in the following way to prevent it from type-checking tests:

1. Install Relevant Packages

   npm install --save-dev ts-jest @types/jest
   

2. Set ts-jest as Transformer in Jest Configuration

   // jest.config.js
   module.exports = {
     // ...
     transform: {
       '^.+\\.tsx?$': [ 'ts-jest', { isolatedModules: true } ],
     },
     // ...
   }
   

   Setting the isolatedModules option to true prevents type-checking when tests are run.

By default, ts-jest will type-check your Jest tests.