How to fix the “Cannot find name ‘it'” error in Jest and TypeScript?

To fix the "Cannot find name ‘it’" error in Jest and TypeScript, we add the ts-jest transform to our test files by adding the entry in jest.config.js.

We install ts-jest by running

npm install jest @types/jest ts-jest

For instance, we write

module.exports = {
  roots: ["<rootDir>/src"],
  transform: {
    "^.+\\.tsx?$": "ts-jest",
  },
  testRegex: "(/__tests__/.*|(\\.|/)(test|spec))\\.tsx?$",
  moduleFileExtensions: ["ts", "tsx", "js", "jsx", "json", "node"],
};

in jest.config.ts to transform "^.+\\.tsx?$" files with ts-jest.

Then in tsconfig.json, we add

{
  "compilerOptions": {
    //...

    "types": ["reflect-metadata", "jest"],
    "typeRoots": ["./types", "./node_modules/@types"]

    //...
  },
  "exclude": ["node_modules", "**/*.spec.ts", "**/*.test.ts"],
  "include": ["./src/**/*.tsx", "./src/**/*.ts"]
}

to add the 'jest' type to the types option.

Now the TypeScript compiler should be aware of Jest types.