Chapter 6. Advanced Types

TypeScript has a world-class type system that supports powerful type-level programming features that might make even the crotchetiest Haskell programmer jealous. As you by now know, that type system isn’t just incredibly expressive, but also easy to use, and makes declaring type constraints and relationships simple, terse, and most of the time, inferred.

We need such an expressive and unusual type system because JavaScript is so dynamic. Modeling things like prototypes, dynamically bound this, function overloads, and always-changing objects requires a rich type system and a utility belt of type operators that would make Batman do a double-take.

I’ll start this chapter with a deep dive into subtyping, assignability, variance, and widening in TypeScript, giving more definition to the intuitions you’ve been developing over the last several chapters. I’ll then cover TypeScript’s control-flow-based typechecking features in more detail, including refinement and totality, and continue with some advanced type-level programming features: keying into and mapping over object types, using conditional types, defining your own type guards, and escape hatches like type assertions and definite assignment assertions. Finally, I’ll cover advanced patterns for squeezing more safety out of your types: the companion object pattern, improving inference for tuple types, simulating nominal types, and safely extending the prototype.

Relationships Between Types

Let’s begin by ...

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