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doc: clarify variable precedence in process.loadEnvFile() #66321

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25 changes: 25 additions & 0 deletions doc/api/process.md
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Original file line number Diff line number Diff line change
Expand Up @@ -2726,6 +2726,13 @@ changes:
Loads the `.env` file into `process.env`. Usage of `NODE_OPTIONS`
in the `.env` file will not have any effect on Node.js.

If a variable is already defined in `process.env`, its existing value takes
precedence and is not overwritten by the value from the file. This also applies
to variables set by a previous call to `process.loadEnvFile()`, so when loading
multiple files, the value from the first file that defines a variable is used.
This differs from passing multiple [`--env-file`][] flags, where values from
later files override those from earlier files.

```cjs
const { loadEnvFile } = require('node:process');
loadEnvFile();
Expand All @@ -2736,6 +2743,22 @@ import { loadEnvFile } from 'node:process';
loadEnvFile();
```

To override existing values, parse the file with [`util.parseEnv()`][] and
assign the result to `process.env`:

```cjs
const { readFileSync } = require('node:fs');
const { parseEnv } = require('node:util');
Object.assign(process.env, parseEnv(readFileSync('.env', 'utf8')));
```

```mjs
import process from 'node:process';
import { readFileSync } from 'node:fs';
import { parseEnv } from 'node:util';
Object.assign(process.env, parseEnv(readFileSync('.env', 'utf8')));
```

## `process.mainModule`

<!-- YAML
Expand Down Expand Up @@ -4632,6 +4655,7 @@ cases:
[`'exit'`]: #event-exit
[`'message'`]: child_process.md#event-message
[`'uncaughtException'`]: #event-uncaughtexception
[`--env-file`]: cli.md#--env-filefile
[`--no-deprecation`]: cli.md#--no-deprecation
[`--permission-audit`]: cli.md#--permission-audit
[`--permission`]: cli.md#--permission
Expand Down Expand Up @@ -4674,6 +4698,7 @@ cases:
[`require.cache`]: modules.md#requirecache
[`require.main`]: modules.md#accessing-the-main-module
[`subprocess.kill()`]: child_process.md#subprocesskillsignal
[`util.parseEnv()`]: util.md#utilparseenvcontent
[`v8.setFlagsFromString()`]: v8.md#v8setflagsfromstringflags
[built-in modules with mandatory `node:` prefix]: modules.md#built-in-modules-with-mandatory-node-prefix
[debugger]: debugger.md
Expand Down
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