Jest demo to demonstrate issue jestjs/jest#2549
By default Jest uses sandbox isolation to prevent tests influencing other tests by changing the global environment. In theory that's a nice thing to have but in reality it is annoying as hell because it breaks important JavaScript internals like instanceof checks. So all tests in this project fail because of this.
To reproduce this scenario disable testEnvironment in jest.config.js and run npm install and npm test. Doesn't matter if you use Jest 24 or 25.
The sandbox isolation can be disabled by specifying a custom test environment
(See src/test/SingleContextNodeEnvironment.js). By using this workaround all tests share the same global
environment. They can influence each other (Which is what Jest tries to prevent) but the big advantage is, that
JavaSCript works again as expected and all tests in this project succeed.
To reproduce this scenario enable testEnvironment in jest.config.js, downgrade jest to 24.9.0 in package.json (Does not work with Jest 25!)
and run npm install and npm test.
Unfortunately the workaround is no longer working with Jest 25. When running the tests then Jest fails with the
error message describe is not defined.
To reproduce this scenaro enable testEnvironment in jest.config.js, upgrade jest to 25.1.0 in package.json
and run npm install and npm test