Watch mode

Instead of manually running npm test all the time we can pass the --watchAll flag to Jest so that it can run on file changes

run npm run test -- --watchAll

note: to pass flags need to use -- two times the first is consumed by npm

From now on Jest will run test cases again on file changes

Jest is smart enough to only re-run test cases that could be affected by the changes to the files (the changed files, all the files that import a changed files, etc.)

note --watch is also available, controls changed files from latest git commit and have a broader list of features, --watch only works inside a git repository

Watch mode menu

By pressing w in the terminal running Jest's in Watch mode we can access a set of new features

› Press f to run only failed tests.
› Press o to only run tests related to changed files.
› Press p to filter by a filename regex pattern.
› Press t to filter by a test name regex pattern.
› Press q to quit watch mode.
› Press Enter to trigger a test run.

Using Watch by default

Is common practice to set npm test script to run Jest in watch mode

{
...
"scripts": {
    "test": "jest --watch"
  },
...
}

Author: Jaga Santagostino