What is a polymorphic 'this' type?

Question presented to candidate: "What is the polymorphic this type in TypeScript? Show a fluent builder where a subclass adds methods, and explain why returning this keeps the chain typed when returning the class name does not."

What a strong answer should cover:

• Inside a class or interface, this used as a type means "the type of the object the member was called on," which is the subclass when called on a subclass instance
• Fluent APIs: a base method declared (): this returns the subclass type, so subclass methods stay available after base-class calls in a chain; an explicit class-name return type loses them
• this also works in parameter positions (equals(other: this)) and in interfaces (tap(fn: (self: this) => void): this)
• this-based type guards (isFile(): this is FileNode) narrow the receiver
• Limits: this behaves like a type parameter, so return new Base() is not assignable to this; cloning needs a prototype copy or similar
• Not to be confused with a this parameter (function f(this: Window)), which types the receiver of a plain function

Code / implementation expected: Yes — a two-level builder whose base methods return this, chained through a subclass method.