They tell us to write tests before, during and after development. But they don't tell us what to do with this forever-running and ever-growing list of tests. If your tests take a double digit number of seconds to execute then you're doing it wrong. Maybe you have already split your tests into fast unit tests that you run all the time, and slow integration tests that you run as needed. Wouldn't it be great to cherry pick those 2-3 integration tests that are relevant to the change you just made and run them? Wouldn't it be great to make unit tests run even faster by executing just those few that are affected by your current work? Test-Creep automatically runs just the subset of tests that are affected by your current work. Best part: This is done with seamlessly Mocha integration so you work as normal.
What is selective test execution?
Selective test execution means running just the relevant subset of your tests instead of all of them. For example, if you have 200 tests, and 10 of them are related to some feature, then if you make a change to this feature you should run only the 10 tests and not the whole 200. test-creep automatically chooses the relevant tests based on istanbul code coverage reports. All this is done for you behind the scenes and you can work normally with just Mocha.
Installation and usage
1. You should use Mocha in your project to run tests. You should use git as a source control.
2. You need to have Mocha installed locally and run it locally rather than globally:
$> ./node_moduels/mocha/bin/mocha ./tests
3. You need to install test-creep:
4. When you run mocha specify to run the special test 'first.js' before all other tests:
first.js is bundled with test-creep and monkey patchs mocha with the required instrumentation (via istanbul).
In addition, it is recommended to add .testdeps_.json to .gitignore (more on this file below).
How does this work?
The first time you execute the command all tests run. first.js monkey patches mocha with istanbul code coverage and tracks the coverage per test (rather than per the whole process). Based on this information test-creep creates a test dependency file in the root of your project (.testdeps_.json). The file specifies for each test which files it uses:
Next time you run the tests (assuming you add first.js to the command) test-creep runs 'git status' to see which files were added/deleted/modified since last commit. Then test-creep searches the dependency file to see which tesst may be affected and instructs mocha to only run these tests. In the example above, if you have uncommited changes only to lib/exceptions.js, then only the first test will be executed.
At any moment you can run mocha without the 'first.js' parameter in which case all tests and not just relevant ones will run.
When to use test-creep?
test-creep sweet spot is in long running test suites, where it can save many seconds or minutes each time you run tests. If you have a test suite that runs super fast (< 2 seconds) then test-creep will probably add more overhead than help. However whenever tests run for more than that test-creep can save you time.
In github or ask me on twitter.