Classes, structural contracts, and private state

The idea

An interface describes capabilities, and a class can implement that shape. Code depending on the interface can accept other compatible implementations without requiring a shared superclass. The implements clause checks the class; it does not change the inferred types of every member body or construct missing methods automatically.

Public structure usually determines compatibility, but private and protected members introduce origin-related restrictions for class compatibility. TypeScript’s private keyword is primarily a checker restriction and is distinct from JavaScript #private fields with runtime enforcement. Neither feature validates an arbitrary external object. Keep mutation behind methods that enforce domain rules, and expose only the observations clients actually need.

interface Reading { value(): number; }
class Fixed implements Reading {
  constructor(private amount: number) {}
  value(): number { return this.amount; }
}
function doubled(reading: Reading): number { return reading.value() * 2; }
console.log(doubled(new Fixed(6)));
console.log(doubled({value: () => 4}));

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8

Both the class instance and the object literal satisfy the required public behavior.