Feature: only run dependent tests on save

🚀 Feature Proposal

Stealing this from mocha-webpack/mochapack’s killer feature in --watch mode:

Unlike mocha, mochapack analyzes your dependency graph and run only those test files that were affected by this file change.

You’ll get continuous feedback whenever you make changes as all tests that are related in any way to this change will be tested again. Isn’t that awesome?

I am proposing the same feature be added to jest under a flag like --onlyOnSave.

Motivation

Running a whole test suite is slow, but immediate feedback on a change is great. Using it.only() and describe.only() gets you closer to speedy test results, but you still need to think about what tests are affected by a change in your source code. This feature reduces all that extra thought so the only thing you need to do is write changes in source and watch the related tests just run.

Example

npx jest --onlyOnSave tests/

in your editor, save a file and watch jest only run the test cases related to said file.

Outline

the implementation would come down to a few things (apologies I dont know the internal workings of jest yet):

on file change:

• build a dependency graph for changed file (find-imports can likely help with this)
• find which dependent files are test files
• execute those test files only

[andykais]

this may be a misunderstanding, I may have not been clear enough in my description. The feature I am describing would change which test files are ran per each file write. E.g., say you have this project structure:

// a.js 

// b.js

// a.test.js
import A from './a'
it('test a', () => {
  ...
})

// b.test.js
import B from './b'
it('test b', () => {
  ...
})

If I boot up mochapack --watch, then in my editor I save a.js, only test file a.test.js will run, if I save file b.js, only test file b.test.js will run.

If you want to see it yourself, then run a simple app with mochapack. Ill do my best to create an example repo later tonight, but its just the standard behavior for mochapack --watch.

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