Ask HN: What are the ways you go about getting comfortable with a new codebase?

Tools like Eclipse's Java Browsing Perspective(https://querix.com/go/beginner/Content/05_workbench/01_ls/02_interface/01_perspectives/java_browsing.htm), Visual Studio's Object Browser make it easier to browse the codebase by components.

What are the ways you do it? Is there anything similar available for functional programming?