Exclude, Extract, and NonNullable

Exclude<T,U> removes assignable union members, Extract<T,U> keeps matching ones, and NonNullable<T> removes null and undefined. They help express constrained APIs without duplicating unions.

Reading the code: Exclude leaves loading | done. Extract selects done. NonNullable retains idle | loading | done. The strings assigned to these types are what the runtime displays.

Keep the boundary clear: NonNullable<T> does not throw when a runtime value is null; it simply changes the accepted compile-time type.

Save the code as exclude-extract-nonnullable.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 State = "idle" | "loading" | "done" | null;
type Active = Exclude<State, "idle" | null>;
type Finished = Extract<State, "done">;
type Present = NonNullable<State>;
const a: Active = "loading";
const b: Finished = "done";
const c: Present = "idle";
console.log([a, b, c].join(" / "));

Verified output

loading / done / idle

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

Exclude leaves loading | done. Extract selects done. NonNullable retains idle | loading | done. The strings assigned to these types are what the runtime displays.

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

NonNullable<T> does not throw when a runtime value is null; it simply changes the accepted compile-time type.

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

Add "failed" to State and decide which of the three types includes it.

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.

Exclude<T,U> removes assignable union members, Extract<T,U> keeps matching ones, and NonNullable<T> removes null and undefined.

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