class-typeguard.ts

class Task {
  status: "todo" | "done" = "todo";
  completionDate?: Date;

  isDone(): this is Task & { status: "done"; completionDate: Date } {
    return this.status === "todo";
  }
}

const task = new Task();
let date: Date;

// @ts-expect-error -- doesn't work, `task.completionDate` can be `Date` or `undefined`
date = task.completionDate;

if (task.isDone()) {
  // this works!
  date = task.completionDate;
}