If you are already using jest-cli, add babel-jest and it will automatically compile JavaScript code using Babel.
yarn add --dev babel-jest @babel/core
If you would like to write your own preprocessor, uninstall and delete babel-jest and set the config.transform option to your preprocessor.
Note: this step is only required if you are using babel-jest with additional code preprocessors.
To explicitly define babel-jest as a transformer for your JavaScript code, map .js files to the babel-jest module. Typescript files are also supported.
"transform": { "^.+\\.[t|j]sx?$": "babel-jest" },