TypeScript notes: types at the data boundary

Type checking happens before runtime

TypeScript catches many structural mistakes, but a type annotation does not inspect a network response or browser storage. An as assertion asks the compiler to trust a claim.

Treat data from storage, forms, or remote services as unknown until it has been checked.

Validate before using

type Note = { id: string; content: string };

function isNote(value: unknown): value is Note {
  if (typeof value !== "object" || value === null) return false;
  return (
    "id" in value &&
    typeof value.id === "string" &&
    "content" in value &&
    typeof value.content === "string"
  );
}

function parseNote(raw: string): Note | null {
  try {
    const value: unknown = JSON.parse(raw);
    return isNote(value) ? value : null;
  } catch {
    return null;
  }
}

This handles both invalid JSON and an unexpected structure. A schema validation library can centralize constraints for larger objects.

Give failure a normal path

Storage may be empty, old records may use different fields, and persistence may be disabled. Handling these as ordinary branches helps the rest of the interface keep working.

Simple types help too

A post summary can explicitly contain its title, date, category, and link. Passing that summary to a component gives its data boundary a clear contract.