Understanding & Explaining Unfamiliar Code
Use AI to onboard into unfamiliar code: trace data flow, decode dense logic, and map a codebase at the right level of detail.
TL;DR
- AI is a fast guide into unfamiliar code: ask it to explain at the level and angle you need.
- Trace the flow ('what happens when X is called?') rather than asking 'what does this do?' in general.
- Verify explanations against the code; the model can describe a plausible version that is not yours.
Set The Level
Your BackgroundTell the model what you already know so it skips or expands accordingly.
"I know JS, new to this repo and to
RxJS. Explain accordingly."Your GoalSay why you are reading it; the purpose shapes the explanation.
"I need to add a field to this form
safely. What must I understand?"Depth DialAsk for a summary first, then detail on demand.
"One-paragraph overview first,
then I'll ask for details."Trace The Flow
Follow An InputPick a concrete scenario and have the model walk the path.
"Trace what happens when submit()
is called with an invalid email."Find Side EffectsAsk what state, network, or storage the code touches.
"List every side effect this
function causes."Map The BranchesHave it enumerate the conditions and what each one does.
"What are all the branches here and
when does each run?"Decode Dense Code
TranslateAsk for a plain-language rewrite of a cryptic block.
"Rewrite this regex/one-liner in
plain steps."Name The PatternHave it identify the design pattern or idiom in use.
"What pattern is this, and why
might it be used here?"AnnotateRequest inline comments that explain the tricky lines.
"Add comments explaining each
non-obvious line."Verify & Map
Spot-CheckConfirm the key claims against the actual source.
Does it really call that? Open the
file and check.Top-Down MapStart with architecture and entry points, then drill in.
"Overview of the architecture and
main entry points first."Note The UnseenRemember it infers anything you did not paste.
"You haven't seen the API layer;
flag guesses about it."Tips
- Set your level: 'explain to someone new to this codebase but fluent in TypeScript'.
- Ask for a step-by-step trace of one concrete input to see the real path through the code.
Warnings
- The model may explain what the code looks like it does, not what it does; spot-check against reality.
- For large systems it only sees what you paste; it will guess about the parts it cannot see.
In Practice
Instead of 'what does this do?', you set your level, pick one real input, and ask the model to trace the path and list side effects, then you spot-check its claims.
- Stating your background keeps the explanation at a useful depth.
- A concrete input makes the model follow the actual control flow.
- Asking for side effects surfaces the risky parts you most need to know.
- A reminder to flag inferences keeps you honest about what it cannot see.
Context: I'm fluent in TypeScript but new to this
codebase. I need to safely change the checkout flow.
Trace, step by step, what happens when
`handleCheckout(cart)` is called with a cart that
contains one out-of-stock item.
For each step, tell me:
- which function is called and what it returns
- any side effects (network, DB writes, events)
- which branch is taken and why
You can only see what I pasted below. Flag any step
where you are inferring behavior you cannot see.
<paste handleCheckout and the functions it calls>FAQ
State your background and goal. 'I know React but not this codebase; explain how auth state flows from login to the protected route' gets a targeted answer. Without a level, you get either a beginner lecture or jargon over your head.
Trace a concrete scenario. 'Walk through what happens, step by step, when a user submits this form' forces the model to follow the real path and surfaces the control flow, side effects, and branches you actually care about.
Treat it as a strong hypothesis, not gospel. The model can produce a confident, plausible explanation of a slightly different version of the code. Spot-check key claims against the source, especially around side effects and edge branches.
Work top-down. Ask for the high-level architecture and the main entry points first, then drill into one module at a time. Paste directory listings and key files so the map is grounded, and note that unseen parts are inferred.