If you'd like to try out Jest with an existing codebase, there are a number of ways to convert to Jest:
If you are using AVA, Chai, Expect.js (by Automattic), Jasmine, Mocha, proxyquire, Should.js or Tape you can use the third-party jest-codemods to do most of the dirty migration work. It runs a code transformation on your codebase using jscodeshift.
To transform your existing tests, navigate to the project containing the tests and run:
npx jest-codemods
More information can be found at https://github.com/skovhus/jest-codemods.
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Licensed under the MIT License.
https://jestjs.io/docs/en/migration-guide