If you use an IDE, learn and exploit its code-exploration features. Use them all the time.
Do global searches whenever you aren't sure how things work. If they aren't fast enough for you, get an SSD. Use them to look for all kinds of things to convince yourself your change is safe.
Add comments as you figure things out. For example, "this algorithm seems to match the one at /src/core/foobar.js:123".
You can also add "DEBT" or "REFACTOR" to your IDE's list of TODO tags, and use it in comments where you see something you think might need cleaning up. If you mark these places now and change some of them later, you'll avoid doing damage; you'll get a chance to learn more before changing things.
Put ticket numbers (Jira, whatever) in your comments too.
Think "dig safe". You're spraying painting warnings near significant opportunities to break things.
Do global searches whenever you aren't sure how things work. If they aren't fast enough for you, get an SSD. Use them to look for all kinds of things to convince yourself your change is safe.
Add comments as you figure things out. For example, "this algorithm seems to match the one at /src/core/foobar.js:123".
You can also add "DEBT" or "REFACTOR" to your IDE's list of TODO tags, and use it in comments where you see something you think might need cleaning up. If you mark these places now and change some of them later, you'll avoid doing damage; you'll get a chance to learn more before changing things.
Put ticket numbers (Jira, whatever) in your comments too.
Think "dig safe". You're spraying painting warnings near significant opportunities to break things.