Learning TypeScript

Chapter 9. Type Modifiers

Types of types from types.

“It’s turtles all the way down,”

Anders likes to say.

By now you’ve read all about how the TypeScript type system works with existing JavaScript constructs such as arrays, classes, and objects. For this chapter and Chapter 10, “Generics”, I’m going to take a step further into the type system itself and show features that focus on writing more precise types, as well as types based on other types.

Top Types

I mentioned the concept of a bottom type back in Chapter 4, “Objects” to describe a type that can have no possible values and can’t be reached. It stands to reason that the opposite might also exist in type theory. It does!

A top type, or universal type, is a type that can represent any possible value in a system. Values of all other types can be provided to a location whose type is a top type. In other words, all types are assignable to a top type.

any, Again

The any type can act as a top type, in that any type can be provided to a location of type any. any is generally used when a location is allowed to accept data of any type, such as the parameters to console.log:

let anyValue: any;
anyValue = "Lucille Ball"; // Ok
anyValue = 123; // Ok

console.log(anyValue); // Ok

The problem with any is that it explicitly tells TypeScript not to perform type checking on that value’s assignability or members. That lack of safety is useful if you’d like to quickly bypass TypeScript’s type checker, but the disabling of type checking reduces ...