What is the this type in TypeScript?

TypeScript provides a special this type that represents the type of the current object in methods and functions, resolved polymorphically. Polymorphic this: in a base class method, returning this typed as this means subclasses automatically get the correct return type: class Builder { setValue(v: string): this { this.value = v; return this; } } — if you subclass Builder and call setValue(), the return type is the subclass type, not just Builder. Enables fluent builder patterns where methods chain correctly through inheritance. this parameter: TypeScript allows declaring an explicit this parameter as the first parameter of a function to specify the required type of this when the function is called: function greet(this: User, greeting: string): void { console.log(greeting + this.name); }. This is a TypeScript-only parameter — it is erased from the compiled output. It prevents calling the function in contexts where this would have the wrong type.