Sourcegraph is an open-source browser extension for GitHub, GitLab, and Bitbucket Server that lets you browse through the code of a public repository.

On top of that, you can also create your own private repository to explore and play in, just like a sandbox.

You can easily find everything that's written in the code, including functions, classes, methods, etc. And if you don't know what a certain piece of code does, you can easily hover over it to find out.

Sourcegraph is a free browser extension.