Beginner’s TypeScript #27

Nhan Nguyen
2 min readJan 19, 2024

Error Message: “Could Not Find A Declaration File”

Inside of the index.ts file, we have a single import:

import Diff from "diff"; // red squiggly line under diff

There is an error under diff that reads:

// hovering over diff shows:
Could not find a declaration file for module 'diff'. Implicitly has an 'any' type.

The diff package is listed in the package.json file, and we can see that it is present inside the node_modules folder. Why is not TypeScript letting us use it?

The issue is that the diff package does not include any TypeScript files, specifically a .d.ts file that contains types.

If we look at the JavaScript files for diff inside of node_modules, we can see that there are no types.

As the error message suggests, the fix for this error is to install the types for the diff package:

npm i --save-dev @types/diff

The @types/diff package is maintained by a group of community members at the DefinitelyTyped GitHub. There are types for many popular JavaScript packages, including diff.

👉 Understanding the Declaration File

Once installed, We will find a new @types directory inside your node_modules, which contains the diff directory and an index.d.ts file.

The index.d.ts file describes the diff module, exporting a namespace, interfaces, and providing helpful comments describing the code. In this case, index.d.ts is not linked to the original repository and has a separate version on npm, but it is maintained in sync with the diff library.

🚀 Using diff

Now that we have the types, the error about lacking the necessary declaration file has gone away. We can use CMD + click to jump to different spots in the diff declaration file inside of node_modules.

DefinitelyTyped and the @types packages provide a way to add type declarations for projects that do not have built-in TypeScript support, making it easier to work with these libraries in our TypeScript projects.

I hope you found it useful. Thanks for reading. 🙏

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