Meta Is Transferring Jest to the OpenJS Foundation
I’m not a fan of Jest, but this is good news anyway. If nothing else, establishing clear dedicated stewardship is a great chance to see if some of its limitations can be overcome.<p>I don’t mean to criticize anyone’s prior work but I know it’s better to be more specific, so my short list of problems I’ve experienced with Jest and reasons I won’t consider it for projects currently:<p>- Its mocking model is coupled to CJS semantics and static analysis and hoisting patterns built on Babel, and it’s hard to work around that especially in legacy code.<p>- Its isolation model is inherently prone to memory leaks and fragile tests, directly related to the above point. If you `import &#x27;anything&#x27;`, in a test module or its dependency tree, you might think you’re initializing once but you’re initializing for each test. If that initialization even exposes a way to teardown it’s just sitting there waiting for you to know you need it in an afterEach, otherwise they run perpetually and never get GCed. Really surprising things like loggers and other seemingly passive libraries which you might develop internally suffer from this. This might all sound like whining, but it can make a large test suite basically useless because you can’t even observe the code that’s still running.<p>- I’ve watched ESM support improve and I’m impressed by the effort, but from my observation the above issues are major barriers to making its mocking and isolation strategies viable. ESM isn’t the reason they’re challenging, but cache invalidation of stateful imports is an exceedingly good way to reveal them.