How do you type third-party libraries?

When working with third-party libraries in TypeScript, typing them correctly is crucial for maintaining type safety, improving developer experience, and preventing runtime errors. However, this can be tricky because not all libraries come with built-in TypeScript definitions, and sometimes the provided types might be incomplete or outdated. Over my years working on various projects, I've encountered many scenarios where understanding how to type third-party libraries effectively made a big difference in code quality and maintainability.

Why Typing Third-Party Libraries Matters

Typing third-party libraries helps catch bugs early, improves IDE autocompletion, and makes refactoring safer. Without proper types, you lose these benefits and might end up with runtime surprises that are hard to debug. For example, if you use a library function incorrectly because you misunderstood its API, TypeScript can warn you upfront if the types are accurate.

On the flip side, blindly trusting third-party types can also cause issues if those types are wrong or incomplete. So, understanding how to handle typing for these libraries is as much about knowing when to trust and when to verify or override types.

Core Ways to Type Third-Party Libraries

There are several approaches to typing third-party libraries, depending on whether the library ships with types or not, and how complex the API is.

1. Use Built-in TypeScript Definitions

Many modern libraries include their own TypeScript definitions. For example, popular libraries like React, lodash, or axios come with type definitions bundled. When you install these packages, TypeScript automatically picks up the types, so you get immediate type safety and autocompletion.

Example:

import axios from 'axios';

axios.get<User>('/api/user/123').then(response => {
  console.log(response.data.name);
});

Here, TypeScript knows the shape of the response because axios ships with types, and you can even specify generics to improve type inference.

2. Use DefinitelyTyped (@types Packages)

If the library doesn’t include types, the next best place to look is the DefinitelyTyped repository. This community-driven project hosts type definitions for thousands of libraries under the @types namespace on npm.

For example, if you want to use moment (which historically didn’t ship with types), you’d install:

npm install moment
npm install --save-dev @types/moment

Then TypeScript picks up the types automatically. This is usually the easiest way to get types for popular libraries without built-in support.

3. Write Your Own Type Declarations

Sometimes, you might use a niche or internal library without any types available. In these cases, you can write your own declaration files (.d.ts) to describe the API surface you use.

For example, if you have a library called my-lib with a function doSomething, you might create a my-lib.d.ts file:

declare module 'my-lib' {
  export function doSomething(input: string): number;
}

This tells TypeScript how to type-check your usage of my-lib. You don’t have to type the entire library upfront—just the parts you use.

4. Use any or unknown as a Last Resort

When you’re in a hurry or the library is too complex to type immediately, you might temporarily use any or unknown to bypass type errors. However, this sacrifices type safety and should be avoided or replaced with proper types as soon as possible.

Real-World Examples and Trade-offs

In one project, I integrated a third-party charting library that didn’t have official TypeScript support. Initially, I used any for the chart options, which worked but made refactoring painful and error-prone. Later, I wrote a minimal declaration file covering the options I used, which improved developer confidence and reduced bugs.

On the other hand, I’ve seen teams blindly install @types packages without verifying their quality. Some DefinitelyTyped definitions are outdated or incomplete, leading to subtle bugs. Always check the type package’s GitHub issues or test the types in a small sandbox before fully adopting them.

Best Practices for Typing Third-Party Libraries

• Prefer built-in types or official @types packages: These are usually well-maintained and tested.
• Write minimal declaration files: Only type what you use to avoid unnecessary overhead.
• Contribute back: If you improve or create types for a popular library, consider contributing to DefinitelyTyped or the library itself.
• Avoid any unless absolutely necessary: It defeats the purpose of TypeScript.
• Keep types up to date: When libraries update, check if their types need updates too.
• Use unknown instead of any when you want to force explicit type checks.

Common Mistakes When Typing Third-Party Libraries

• Assuming types are always correct: Types can be wrong or incomplete, especially in community-maintained packages.
• Typing everything upfront: Over-typing can slow down development. Focus on the parts you actually use.
• Ignoring type errors: Suppressing errors with @ts-ignore without understanding the root cause can hide real issues.
• Not version-locking types: Mismatched library and type versions can cause confusing errors.

Performance and Scalability Considerations

Typing third-party libraries doesn’t directly affect runtime performance, but it impacts developer productivity and codebase scalability. Well-typed libraries help teams scale by reducing bugs and making onboarding easier. On the other hand, overly complex or inaccurate types can slow down the TypeScript compiler and confuse developers.

When writing your own declaration files, keep them as simple as possible to avoid bloated type checks. Also, avoid deep recursive types or excessive generics unless necessary, as these can degrade compile times.

Security Considerations

While typing itself doesn’t improve runtime security, it helps prevent certain classes of bugs that can lead to vulnerabilities. For example, if you mistype a function parameter or ignore return types, you might introduce injection flaws or logic errors. Accurate types encourage safer coding patterns and better validation.

Also, when using third-party libraries, always audit their source and dependencies for security risks. Types won’t protect you from malicious code or supply chain attacks.

Interview Tips for Discussing Typing Third-Party Libraries

• Explain the different sources of types: built-in, DefinitelyTyped, and custom declarations.
• Talk about trade-offs between quick fixes (any) and long-term maintainability.
• Mention real-world scenarios where typing helped catch bugs or improved refactoring.
• Show awareness of common pitfalls, like outdated types or version mismatches.
• Demonstrate knowledge of TypeScript tooling, like tsc, tsconfig.json, and IDE integration.

Comparison Table: Typing Approaches for Third-Party Libraries

Approach	Pros	Cons	When to Use
Built-in Types	Accurate, maintained by library authors, seamless integration	Only available for some libraries	Preferred for popular, actively maintained libraries
@types Packages (DefinitelyTyped)	Wide coverage, community maintained, easy to install	May be outdated or incomplete, version mismatch risks	When no built-in types exist but community support is available
Custom Declaration Files	Flexible, tailored to your usage, no external dependencies	Requires manual maintenance, can be time-consuming	For internal or niche libraries without existing types
any / unknown	Quick workaround, no upfront typing effort	Loss of type safety, potential runtime bugs	Temporary solution during prototyping or legacy migration

Practical Production Scenario

Imagine you’re integrating a payment gateway SDK that only ships as a JavaScript package with no TypeScript support. You want to use it in a large codebase with strict type checking enabled.

Here’s a practical approach:

1. Check if there’s an official or community @types package. If yes, install and test it.
2. If not, write minimal declaration files for the parts of the SDK you use, focusing on core functions like initialize, processPayment, and event handlers.
3. Use unknown for complex return types initially, then refine as you understand the API better.
4. Gradually expand the types as the integration grows, improving safety without blocking development.
5. Document any assumptions or gaps in the types for future maintainers.

This approach balances safety, maintainability, and developer velocity, which is often the sweet spot in real projects.