type StudentId = number | string;

interface Course {
  title: string;
  finished: boolean;
}

interface Student {
  id: StudentId;
  name: string;
  age?: number;
  courses: Course[];
}

function pickFirst<T>(list: T[]): T {
  return list[0];
}

function formatStudent(student: Student): string {
  const courseCount = student.courses.length;
  return `${student.name} 当前有 ${courseCount} 门课程，编号是 ${student.id}`;
}

const student: Student = {
  id: "stu-1",
  name: "林晨",
  courses: [
    { title: "基本类型", finished: true },
    { title: "接口", finished: false },
  ],
};

const firstCourse = pickFirst(student.courses);

console.log(formatStudent(student));
console.log(firstCourse);