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zbjornson/jest-native-idempotent.js

Last active March 10, 2019 19:59
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Jest gotchas
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jest-native-idempotent.js
/**
* Jest's require() cache is reset between tests (modules are re-evaluated)
* for most, but not all modules. In the "not all" group are native modules,
* so all interactions with them must be idempotent.
* https://github.com/facebook/jest/issues/4413
*
* Encountered in https://github.com/Automattic/node-canvas/issues/1310,
* https://github.com/Automattic/node-canvas/issues/1294,
* https://github.com/Automattic/node-canvas/issues/1250
*/
const nativemodule = require("build/Release/something.node");
// next two lines are not idempotent
const privateMethod = nativemodule.doSomething;
delete nativemodule.doSomething;
// (privateMethod is used here for something)
module.exports = nativemodule;
Raw
jest-process-cached.js
/**
* In the same vein as above, Node.js's core modules like `process` are in
* the "not all" group so e.g. attaching event listeners creates leaks and
* other unexpected behaviors.
*
* Encountered in https://github.com/siimon/prom-client/pull/147#issuecomment-326782465
*/
process.on("message", msg => {
// a new listener is attached every time this module is required
// and jest has reset its require() cache.
});
@zbjornson
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zbjornson commented Feb 1, 2019 •
edited

Other possible issues similar to above: modules that open file descriptors, open sockets or set timeouts/intervals.

Additionally, jest relies on some properties being configurable. Breakages this has caused:

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