Effective TypeScript, 2nd Edition

Chapter 8. Type Declarations and @types

Dependency management can be confusing in any language, and TypeScript is no exception. In fact, because types are often shipped as separate packages, dependencies in TypeScript can be especially bewildering.

This chapter will help you build a mental model for how dependencies work in TypeScript and show you how to sort through some of the issues that can come up with them. It will also help you craft your own type declaration files to publish and share with others. By writing great type declarations, you can help not just your own project but the entire TypeScript community.

Item 65: Put TypeScript and @types in devDependencies

The Node Package Manager, npm, is ubiquitous in the JavaScript world. It provides both a repository of JavaScript libraries (the npm registry) and a way to specify which versions of them you depend on (package.json).

npm draws a distinction between a few types of dependencies, each of which goes in a separate section of package.json:

dependencies

These are packages that are required to run your JavaScript. If you import lodash at runtime, then it should go in dependencies. When you publish your code on npm and another user installs it, it will also install these dependencies. (These are known as transitive dependencies.)

devDependencies

These packages are used to develop and test your code but are not required at runtime. Your test framework is an example of a devDependency. Unlike dependencies, these are