Design by Contract in OOP with TypeScript

What Is a Contract? A contract is a set of rules that defines how two parties are expected to interact. It answers three questions: what is allowed as input; what must be produced as output; who is responsible for what. In real life, this is easy to understand through the example of an employment agreement. An employee agrees to work during the agreed hours, and the employer agrees to pay a salary. If both sides follow the terms, the relationship remains predictable. If not, the contract is broken. The same idea applies to programming. One module calls another and expects certain behavior. In order for the system to remain reliable, the rules must be clear in advance: what data may be passed in; what result should be produced; what conditions must always remain true. That is exactly what Design by Contract is about. Contracts in Programming In practice, many bugs do not come from complicated logic, but from implicit assumptions. One module assumes it will receive valid data. Another as