Effective TypeScript

Chapter 3. Type Inference

For programming languages used in industry, “statically typed” and “explicitly typed” have traditionally been synonymous. C, C++, Java: they all made you write out your types. But academic languages never conflated these two things: languages like ML and Haskell have long had sophisticated type inference systems, and in the past decade this has begun to work its way into industry languages. C++ has added auto, and Java has added var.

TypeScript makes extensive use of type inference. Used well, this can dramatically reduce the number of type annotations your code requires to get full type safety. One of the easiest ways to tell a TypeScript beginner from a more experienced user is by the number of type annotations. An experienced TypeScript developer will use relatively few annotations (but use them to great effect), while a beginner may drown their code in redundant type annotations.

This chapter shows you some of the problems that can arise with type inference and how to fix them. After reading it, you should have a good understanding of how TypeScript infers types, when you still need to write type declarations, and when it’s a good idea to write type declarations even when a type can be inferred.

Item 19: Avoid Cluttering Your Code with Inferable Types

The first thing that many new TypeScript developers do when they convert a codebase from JavaScript is fill it with type annotations. TypeScript is about types, after all! But in TypeScript many annotations ...