TypeScript is OSS and on GitHub and the team welcomes community input.


Super easy:

git clone
cd TypeScript
npm install -g jake
npm install

Setup Fork

You would obviously need to setup Microsoft/TypeScript as an upstream remote and your own fork (use the GitHub fork button) as origin:

git remote rm origin
git remote rm upstream
git remote add upstream
git remote add origin

Additionally I like to work off branches like bas/ to have it show up cleaner in the branch listings.

Running Tests

There are lots of test and build options in their JakeFile. You can run all tests with jake runtests


Baselines are used to manage if there are any changes in the expected output of the TypeScript compiler. Baselines are located in tests/baselines.

  • Reference (expected) baselines: tests/baselines/reference
  • Generated (in this test run) baselines : tests/baselines/local (this folder is in .gitignore)

If there are any differences between these folders tests will fail. You can diff the two folders with tools like BeyondCompare or KDiff3.

If you think these changes in generated files are valid then accept baselines using jake baseline-accept. The changes to reference baselines will now show as a git diff you can commit.

Note that if you don't run all tests then use jake baseline-accept[soft] which will only copy over the new files and not delete the whole reference directory.

Test Categories

There are different categories for different scenarios and even different test infrastructures. Here are a few of these explained.

Compiler Tests

These ensure that compiling a file :

  • generates errors as expected
  • generated JS as expected
  • types are identified as expected
  • symbols are identified as expected

These expectations are validated using the baselines infrastructure.

Creating a Compiler Test

Test can be created by adding a new file yourtest.ts to tests/cases/compiler. As soon as you do so and run the tests you should get baseline failure. Accept these baselines (to get them to show up in git), and tweak them to be what you expect them to be ... now get the tests to pass.

Run all of these in isolation using jake runtests tests=compiler, or just your new file using jake runtests tests=compiler/yourtest

I will even often do jake runtests tests=compiler/yourtest || jake baseline-accept[soft] and get the diff in git.

Debugging Tests

jake runtests-browser tests=theNameOfYourTest and debugging in-browser usually works pretty well.

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