Design by Contract and JS

Design by contract (DbC) is an approach in application design, which originally came from Eiffel, but now widely used on different languages (particularly in Java). In real world one party (supplier) makes an offer for an agreement (business contract) and another one (client) accepts. The same way can be described relations between objects in software engineering. As a declared agreement is accepted by the client object, the last one is expected to keep its rules.