A user could express 2 * 5 + 1 as

Because TypeScript used to infer the type BasicCalculator for each method in BasicCalculator that returned this, the type system would forget that it had whenever using a BasicCalculator method.

This is no longer the case - TypeScript now infers this to have a special type called this whenever inside an instance method of a class.The type is written as so, and basically means “the type of the left side of the dot in a method call”.