Here are 3 tips to learn a new codebase fast:

1. Start running the code ASAP. Add a log statement to observe when it gets hit and the state of the variables at that point.

DON'T get stuck reading documentation about the language or codebase.

2. Intentionally break things and understand *how* they break. Instead of incrementing something by one, increment it by two. Can you explain the result of that change?

Understanding the current code is a prerequisite for making your code change.

3. Land a low-effort code change quickly. Fix a spelling error, modify a comment, or add a unit test.

It'll be easier to do your "real" work once you're familiar with the workflow of landing code (and you'll get a confidence boost!)

Courtesy:  Rahul Pandey : Android Lead at Facebook, Stanford Lecturer