process.loadEnvFile() does not override existing environment variables

Version

v24.14.0

Platform

Darwin telsha 25.4.0 Darwin Kernel Version 25.4.0: Thu Mar 19 19:31:09 PDT 2026; root:xnu-12377.101.15~1/RELEASE_ARM64_T8132 arm64

Subsystem

No response

What steps will reproduce the bug?

1. Create two files: .env with KEY=original and .env.test with KEY=override.
2. Run the following in the Node.js REPL:

// Load the first file
process.loadEnvFile(".env");
console.log(process.env.KEY); // 'original'

// Attempt to load the second file to override the value
process.loadEnvFile(".env.test");
console.log(process.env.KEY); // Still 'original'

Mitigation:

import { parseEnv } from 'node:util';
import { readFileSync } from 'node:fs';

// This forces the override
const integConfigs = parseEnv(readFileSync("apps/server/.env.integ", "utf8"));
Object.assign(process.env, integConfigs);

How often does it reproduce? Is there a required condition?

It reproduces 100% of the time. The condition is that a variable must already exist in process.env (either from the shell or a previous loadEnvFile call) for the "bug" to be visible.

What is the expected behavior? Why is that the expected behavior?

While the default behavior of not overriding is standard for dotenv, there is currently no programmatic way (via options) to force an override using process.loadEnvFile().

At the very least, subsequent calls to process.loadEnvFile() should support an { override: true } option to allow developers to switch environment contexts (e.g., switching to an integration/test config) without manually parsing files via util.parseEnv.

What do you see instead?

The variable remains locked to the first value it was assigned. Subsequent calls to loadEnvFile for the same key are silently ignored for that specific key.

Additional information

Most modern environment loaders (like the dotenv npm package) provide an override flag. Without this, process.loadEnvFile is significantly less useful for complex test runners or mono-repos where multiple .env files must be composed or swapped during a single process lifetime.

Start by reproducing the behavior with process.loadEnvFile() in the Node.js REPL using the two .env files described in the issue. Read the process.loadEnvFile() API and its existing environment-variable handling; done means a documented override option can replace existing values while preserving the default behavior, with tests covering repeated loads.