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 Jasmine, or a Jasmine like API (for example Mocha), Jest should be mostly compatible, which makes it less complicated to migrate to.
• If you are using AVA, Expect.js (by Automattic), Jasmine, Mocha, proxyquire, Should.js or Tape you can automatically migrate with Jest Codemods (see below).
• If you like chai, you can upgrade to Jest and continue using chai. However, we recommend trying out Jest's assertions and their failure messages. Jest Codemods can migrate from chai (see below).

jest-codemods

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.