TypeScript — Into the Unknown

An introduction to the unknown type: a better any

Photo by Nicolas Lobos on Unsplash.

TypeScript code around the world is littered with the any type, and it drives me absolutely crazy. There is no (valid) reason to use it in application code, and you are opting out of the safety of a static type system.

Many developers who are new (or not so new to TypeScript) assume that when you don’t know what type you are going to get, you should type it with any. TypeScript’s official documentation also tells you that this is not the case:

“Don’t use any as a type unless you are in the process of migrating a JavaScript project to TypeScript. The compiler effectively treats any as “please turn off type checking for this thing”. It is similar to putting an @ts-ignore comment around every usage of the variable. This can be very helpful when you are first migrating a JavaScript project to TypeScript as you can set the type for stuff you haven’t migrated yet as any, but in a full TypeScript project you are disabling type checking for any parts of your program that use it.”

Unless you are adding type definitions to an open source JavaScript library, do not use any. If you are maintaining an open source JS library, then I appreciate you adding type definitions at all and understand the time constraints you are under. If throwing in an any here and there allows you to add types to 90% of your API, then I think this is a valid use case. I hope they are all refactored to unknown one day when time permits. Ideally, members of the TypeScript community could make PRs and help refactor these for you.

What if I want to accept anything or don’t know the type I’ll get? Again, TypeScript’s documentation comes to the rescue:

“In cases where you don’t know what type you want to accept, or when you want to accept anything because you will be blindly passing it through without interacting with it, you can use unknown.”

The unknown Type

In TypeScript 3.0, we got the unknown type, which let us signal “This can be any value, so you must perform some type checking before you use it.” Unlike with any, you cannot access any properties on values with the type unknown or call/construct them. unknown forces us to safely introspect…