Partial, Required, and Readonly

Partial<T> allows omitted properties; Required<T> requires declared properties; Readonly<T> prevents typed assignment through that view. They can be composed when editing a record in stages, but none of them performs runtime validation.

Reading the code: changes may omit id. full supplies both id and nickname, satisfying Required. safe forbids writes through that reference during type checking; console.log reads its values normally.

Keep the boundary clear: Required does not fill in missing values automatically; it only reports errors if an assigned value omits required keys.

Save the code as partial-required-readonly.ts. The result below was captured after compiling this exact example with strict TypeScript checking and executing the emitted JavaScript in Node.js.

TypeScript code

type Profile = { id: number; nickname?: string };
const changes: Partial<Profile> = { nickname: "Lin" };
const full: Required<Profile> = { id: 5, nickname: changes.nickname ?? "anonymous" };
const safe: Readonly<Profile> = full;
console.log(`${safe.id}:${safe.nickname}`);

Verified output

5:Lin

Build: npx tsc --strict --target es2022 --module commonjs --lib es2022,dom,esnext.disposable partial-required-readonly.ts; run: node partial-required-readonly.js. The exact filename may be changed freely. When a multi-file project exists, prefer its tsconfig.json and build scripts.

changes may omit id. full supplies both id and nickname, satisfying Required. safe forbids writes through that reference during type checking; console.log reads its values normally.

The output shown above comes from the initialized values in this program. Replacing a type annotation alone does not transform a JavaScript value.

Required does not fill in missing values automatically; it only reports errors if an assigned value omits required keys.

Check both the TypeScript diagnostic and the emitted JavaScript behavior. A program can compile correctly and still need runtime validation or error handling.

Make a Partial<Profile> containing only id, then construct a Required<Profile> with a default nickname.

Before running your change, write down the type you expect TypeScript to infer and the output you expect JavaScript to print. Then check both predictions.

Partial<T> allows omitted properties; Required<T> requires declared properties; Readonly<T> prevents typed assignment through that view.

Try the practice change and re-run the exact program to test your understanding.