Entities and value objects

📘What it really is

• Entities have an identity that lasts over time, even as their details change: a Student, a Tutor, a Booking. Two bookings with identical details are still two different bookings; they're compared by ID.

• Value objects are defined only by their values, have no identity, and are immutable (never changed; replaced instead): Money (amount and currency), TimeRange (start and end), EmailAddress, PhoneNumber, Subject.

• Value objects carry rules and make wrong states impossible:

export class Money {
  readonly paise: number;
  constructor(paise: number) {
    if (!Number.isInteger(paise) || paise < 0) throw new Error("Invalid amount");
    this.paise = paise;
  }
  add(other: Money): Money { return new Money(this.paise + other.paise); }
  format(): string { return `₹${(this.paise / 100).toFixed(2)}`; }
}

export class TimeRange {
  readonly start: Date;
  readonly end: Date;
  constructor(start: Date, end: Date) {
    if (end <= start) throw new Error("End must be after start");
    this.start = start;
    this.end = end;
  }
  overlaps(other: TimeRange): boolean {
    return this.start < other.end && other.start < this.end;
  }
}

• Storing money as whole paise (integers) avoids floating-point mistakes (Module 15); a TimeRange can't be backwards; an EmailAddress is always valid once created. Rules live in one place, not scattered as checks around the code.

• Avoid "primitive obsession": passing raw numbers and strings everywhere (price: number, start: string), where any value, including wrong ones, can slip in.

• Time zones: store times in UTC, display in the user's zone (Asia/Kolkata), and keep the zone in the TimeRange or alongside it where it matters.