Tip to Familiarize with New JavaScript Codebases

Updated May 26, 2024
Created April 29, 2021

Please read the origninal post on https://www.kevinpeters.net/the-fastest-way-to-understand-new-code-bases.

The following are direct snippets from the above link.


React.js is a frontend framework that almost every web developer knows by now. What most people do not know is how the codebase is structured and what are the core parts. So let us have a look at it.

npx code-complexity . --limit 20 --sort ratio

Running the command will lead to the following result:

file complexity churn ratio
packages/eslint-plugin-react-hooks/**tests**/ESLintRuleExhaustiveDeps-test.js 7742 51 394842
packages/react/src/**tests**/ReactProfiler-test.internal.js 4002 95 380190
packages/react-reconciler/src/ReactFiberWorkLoop.new.js 2373 139 329847
packages/react-reconciler/src/ReactFiberWorkLoop.old.js 2373 114 270522
packages/react-dom/src/server/ReactPartialRenderer.js 1379 122 168238
packages/react-reconciler/src/ReactFiberCommitWork.new.js 2262 71 160602
packages/react-devtools-shared/src/backend/renderer.js 2952 54 159408
packages/react-reconciler/src/ReactFiberBeginWork.new.js 2903 53 153859
scripts/rollup/bundles.js 760 199 151240
packages/react-reconciler/src/ReactFiberHooks.new.js 2622 56 146832
packages/react-dom/src/client/ReactDOMHostConfig.js 1018 140 142520
packages/react-reconciler/src/ReactFiberHooks.old.js 2622 50 131100
packages/react-reconciler/src/**tests**/ReactHooks-test.internal.js 1641 74 121434
packages/react-dom/src/**tests**/ReactDOMComponent-test.js 2346 51 119646
packages/react-dom/src/**tests**/ReactDOMServerPartialHydration-test.internal.js 2150 49 105350
packages/react-noop-renderer/src/createReactNoop.js 966 109 105294
packages/react-reconciler/src/ReactFiberCommitWork.old.js 2262 46 104052
packages/react-reconciler/src/ReactFiberBeginWork.old.js 2903 35 101605
packages/react-reconciler/src/**tests**/ReactIncrementalErrorHandling-test.internal.js 1532 62 94984
packages/react-refresh/src/**tests**/ReactFresh-test.js 3165 29 91785

What we can see here is that two sub-packages are probably the most interesting to understand:

Understanding React Fiber and how react-dom's partial renderer is working will give you a good idea about React's architecture. A good thing about the code within React is that it is well documented with comments even though it is complex at first.

Where does this technique come from?

This technique of analyzing a codebase originally came from a book that handles refactoring large codebases via a process: Software Design X-Rays by Adam Tornhill. It is a great book and teaches you a lot of ways to structure your code and what parts are worth refactoring. A great book. I think every software engineer should have read it at some point because it will help them to understand a codebase differently. With working on a project, people will get familiar with different parts of the software and of course, they will have their special "area" of code where they are super comfortable. If this code is good and understandable is another question though, that this book tries to answer.