Creating types for process.env with Zod

In this blog post, we will explore how to use Zod, a powerful TypeScript-first schema validation library, to create globally defined TypeScript types for environment variables in a project. Let’s get straight to the code.

environment.d.ts

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import { z } from "zod";

const environmentVariables = z.object({
	OPENAI_API_KEY: z.string(),
	NEXT_PUBLIC_SUPABASE_URL: z.string(),
	SUPABASE_SERVICE_ROLE_KEY: z.string(),
});

declare global {
	namespace NodeJS {
		interface ProcessEnv extends z.infer<typeof environmentVariables> {}
	}
}

First we define environmentVariables, a Zod schema. This schema specifies the shape and validation rules for a set of environment variables that your application requires. In this case each of these variables is expected to be of type string.

The next part of the code introduces a global declaration. Here’s a breakdown of the code:

declare global: This is a TypeScript declaration that allows us to modify or extend global types.

namespace NodeJS: The NodeJS namespace provides a way to declare types specifically for Node.js environments.

interface ProcessEnv extends z.infer {}: This line extends the ProcessEnv interface and sets it to inherit the inferred type from typeof environmentVariables.

Important part here is that by extending the ProcessEnv interface with z.infer, we are essentially stating that the environment variables should conform to the structure and validation rules specified in the environmentVariables Zod schema. And this will provide type safety and validation when accessing environment variables throughout the project.