The Prepack-powered React compiler prototype is in active development. There are a few scripts you can use to debug issues in it. Note that it assumes a Facebook-specific JavaScript environment, and is neither generally usable nor production-ready.

Typically, the input should look similar to this:

require('react');

__evaluatePureFunction(function() {

  // More code here

  function App() {
    // This is a React component,
    // it can be a function or a class.
  }

  __optimizeReactComponentTree(App);

  module.exports = App;

});

All React component definitions should be inside that __evaluatePureFunction() closure.

Some components may be marked with a __optimizeReactComponentTree() call. Those will be passed to the React reconciler. Optionally, you can pass { firstRenderOnly: true } as a second argument to evaluate the tree in a special first render mode.

Save the input code as fb-www/input.js. This file is gitignored.

Then run yarn debug-fb-www.

If the compilation is successful, the result will be saved to fb-www/output.js.

If you use Visual Studio Code, create a file called .vscode/launch.json:

{
    "version": "0.2.0",
    "configurations": [
        {
            "type": "node",
            "request": "launch",
            "name": "Debug fb-www",
            "program": "${workspaceFolder}/scripts/debug-fb-www.js"
        }
    ]
}

After you create it, open the prepack root folder in VS Code, and switch to the “Debugger” pane.

You will see a green button next to “Debug fb-www” command. You can start the debugger by pressing it. It will also use fb-www/input.js as the input, and write the result to fb-www/output.js.

The debugger is not always reliable with setting breakpoints, so the most reliable way to pause is to write code like

if (someCondition) {
  debugger;
}

and restart the debugger. This will cause it to pause on this line.

Typically, you’ll want to run yarn watch in the terminal so that any changes you made in the editor are immediately compiled.

You can run the React tests from terminal with yarn test-react.

To enter a watching mode, run yarn test-react --watch. This will re-run them on any change. This can be pretty slow.

If you’re debugging a specific test case, the easiest way to focus on it is to:

• Open test/react/SomeTestFileName-test.js.

• Find the test in the code by searching for its filename.

  For example, you may find something like:

      it("fb-www 5", () => {
        runTest(directory, "fb5.js");
      });

• Change it to fit to “focus” on a specific test and skip all other tests.

  -    it("fb-www 5", () => {
  +    fit("fb-www 5", () => {
         runTest(directory, "fb5.js");
      });

• Run the watch mode: yarn test-react --watch SomeTestFileName

Now only this test alone will re-run on every change which should help debug problems faster.

By default, tests run in four different input/output configurations. If too many runs are confusing when debugging a problem, you can run a subset with yarn test-react-fast --watch SomeFileName. Unlike yarn test-react, yarn test-react-fast skips checking that JSX syntax works. This mode is much faster but you will see some annoying messages about obsolete snapshots (which you should ignore in this mode).

Finally, sometimes it’s helpful to see the code Prepack is emitting. Search for a variable called transformedSource a file called setupReactTests.js. If you add console.log(transformedSource) you will see the Prepack output during test runs.

Don’t forget to revert any such changes before committing!