Explaining & Reviewing Code
Paste code and ask Claude to explain what it does or to review it for bugs and improvements.
Two of the highest-value uses of Claude are understanding unfamiliar code and getting a second pair of eyes on your own.
Explain
Paste a function and ask Claude to walk through it line by line, name the tricky parts, and describe edge cases. This is faster than reading docs for a library you have never seen.
Review
Ask Claude to review a diff or a file for bugs, security issues, and readability. Be explicit about what you care about: "focus on correctness and security, not style".
Make reviews actionable
Ask Claude to report each finding with a short explanation and a suggested fix, so you can act on it immediately.
Example
const code = `function getTotal(items) {
let total = 0;
for (let i = 0; i <= items.length; i++) {
total += items[i].price;
}
return total;
}`;
const prompt = `Review this JavaScript for bugs. For each issue, give the problem\nand a corrected version. Focus on correctness.\n\n${code}`;
const response = await client.messages.create({
model: "claude-opus-4-8",
max_tokens: 2048,
messages: [{ role: "user", content: prompt }],
});
// Claude will flag the off-by-one bug: i <= items.length should be i < items.lengthWhen to use it
- Paste an unfamiliar open-source utility function and ask Claude to explain each line before you use it in production.
- Have Claude review a pull request diff for potential race conditions or unhandled promise rejections.
- Ask Claude to spot security issues in an authentication flow before it ships to users.
More examples
Ask Claude to explain a function
Sends unfamiliar code and asks for a line-by-line explanation to accelerate understanding.
const code = `
function debounce(fn, delay) {
let timer;
return (...args) => {
clearTimeout(timer);
timer = setTimeout(() => fn(...args), delay);
};
}`;
const msg = await client.messages.create({
model: "claude-haiku-4-5",
max_tokens: 256,
messages: [{ role: "user", content: `Explain this JS function line by line:\n${code}` }],
});Code review for bugs and improvements
Uses a senior-engineer persona in the system prompt to get direct, actionable review feedback.
const code = `/* ...paste your function here... */`;
const msg = await client.messages.create({
model: "claude-sonnet-4-5",
max_tokens: 512,
system: "You are a senior JavaScript engineer. Be direct and concise.",
messages: [{
role: "user",
content: `Review this code for bugs, edge cases, and readability improvements:\n${code}`
}],
});Security review of an auth handler
Points Claude at a deliberately flawed handler to get a focused list of security vulnerabilities.
const handler = `
async function login(req, res) {
const { email, password } = req.body;
const user = await db.query(
'SELECT * FROM users WHERE email = ' + email
);
if (user && user.password === password) res.json({ token: 'abc' });
}`;
const msg = await client.messages.create({
model: "claude-sonnet-4-5",
max_tokens: 512,
messages: [{ role: "user", content: `List all security issues in this login handler:\n${handler}` }],
});
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