Checkpoint: Public Records

A service stores a full profile, but the report should expose only id and displayName. The full profile also has an internal numeric score and a private note. The public function receives trusted Profile data and returns a fresh PublicProfile with exactly the two public keys.

This is projection, not validation of unknown input. Do not silently broaden the task into a universal sanitizer. Input parsing will get its own typed boundary on Day 4. Today the important distinction is that a narrower type annotation does not strip properties.

Test two profiles, compare keys, and verify the original data remains unchanged. Returning a smaller view can be useful for caller APIs, but actual privacy also depends on which real data you transmit; the runtime object matters.

Predict, run, and explain

Write the output and trace the important state changes before checking it. Then run the program. Use Check types first; execution is allowed only when the real compiler accepts the source.

const full = { id: "p1", displayName: "Ada", note: "private" };
const publicView = { id: full.id, displayName: full.displayName };

console.log(JSON.stringify(publicView));

{"id":"p1","displayName":"Ada"}

Acceptance cases before you code

Write a small test plan before implementing public profile projection. These cases extend the brief; they are not a second worked solution. Predict the expected behavior independently, then test through the public function. A good ordinary example and a good boundary example should catch different possible mistakes.

After the core task passes, deliberately break one rule and identify the case that notices it. Restore the implementation and explain the failure in terms of the contract, not the number of assertions. If a case requires a concept you have not recalled, revisit the relevant lesson before opening the solution.

Your turn

// Use an explicit returned object, not a whole-input spread.

type Profile = {
  id: string;

  displayName: string;

  score: number;

  note: string;
};

type PublicProfile = Pick<Profile, "id" | "displayName">;

function projectProfile(profile: Profile): PublicProfile {
  return { id: profile.id, displayName: profile.displayName };
}

const original: Profile = {
  id: "p1",
  displayName: "Ada",
  score: 10,
  note: "private"
};
const result = projectProfile(original);

if ("note" in result) {
  throw new Error("Projection leaked note");
}

console.log(result.id, result.displayName);
console.log(Object.keys(result).join(","));
console.log(original.note === "private");

p1 Ada
id,displayName
true

Change it and test it

Mutate the function to return the original object and confirm the runtime assertion fails even though checking passes. Restore projection. Explain whether this function would be safe to call with an unvalidated JSON value.