How do I type process.env variables in JavaScript?

The short answer

Read every variable in one config module, check it there, and export plain typed values. The rest of the program imports the config and never touches process.env:

/**
 * @param {string} name
 * @returns {string}
 */
function required(name) {
  const value = process.env[name];
  if (value === undefined || value === "") throw new Error(`Missing environment variable ${name}`);
  return value;
}

export const config = {
  databaseUrl: required("DATABASE_URL"),
  port: Number(process.env.PORT ?? 3000),
  debug: process.env.DEBUG === "1",
};

console.log(new URL(config.databaseUrl).host, config.port.toFixed(0), config.debug);

function required(name: string): string {
  const value = process.env[name];
  if (value === undefined || value === "") throw new Error(`Missing environment variable ${name}`);
  return value;
}

export const config = {
  databaseUrl: required("DATABASE_URL"),
  port: Number(process.env.PORT ?? 3000),
  debug: process.env.DEBUG === "1",
};

console.log(new URL(config.databaseUrl).host, config.port.toFixed(0), config.debug);

required() returns a string because it throws otherwise, so config.databaseUrl is a string everywhere; port is a number and debug a boolean.

Why it works

@types/node declares process.env as an object whose values are string | undefined, which is the truth: any variable may be missing. Checking once and throwing early turns that uncertainty into a startup error with a clear message, instead of a TypeError deep in the program. tsc follows the check: after value === undefined throws, value is a string.

The same thing in TypeScript

The TypeScript equivalent tab is the same module with the parameter and return type written inline. Nothing about process.env changes between the two.

What goes wrong

Passing a variable straight to a function that needs a string fails, because it may be undefined (TS2345):

/** @param {string} url */
function connect(url) {
  return new URL(url);
}

connect(process.env.DATABASE_URL);error TS2345: Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
  Type 'undefined' is not assignable to type 'string'.

You can also declare the variables on NodeJS.ProcessEnv in a .d.ts file. That types DATABASE_URL as a string, but it is only a declaration: nothing checks that the variable is set, so a missing one surfaces as undefined at runtime. Optional ones stay string | undefined:

/** @param {string} url */
function connect(url) {
  return new URL(url);
}

connect(process.env.DATABASE_URL);
connect(process.env.PORT);error TS2345: Argument of type 'string | undefined' is not assignable to parameter of type 'string'.
  Type 'undefined' is not assignable to type 'string'.

Gotchas

• Only strings go in. process.env.PORT = 5 is TS2322: Node.js would store the text "5", and the types say so. Write String(5).
• process.env.X ?? "default" is a string. A default with ?? narrows the type the same way a check does.
• Browsers have no process. In front-end code, bundlers commonly replace process.env.X or import.meta.env.X at build time; declare those names the way the globals recipe does.