src: don't overwrite environment from .env file

src/node_dotenv.cc

@@ -48,6 +48,10 @@ void Dotenv::SetEnvironment(node::Environment* env) {
   for (const auto& entry : store_) {
     auto key = entry.first;
     auto value = entry.second;
+
+    auto existing = env->env_vars()->Get(key.data());
+
+    if (existing.IsNothing()) {
       env->env_vars()->Set(
           isolate,
           v8::String::NewFromUtf8(
@@ -58,6 +62,7 @@ void Dotenv::SetEnvironment(node::Environment* env) {
               .ToLocalChecked());
     }
   }
+}
 
 void Dotenv::ParsePath(const std::string_view path) {
   uv_fs_t req;

test/fixtures/dotenv/valid.env

@@ -31,5 +31,5 @@ RETAIN_INNER_QUOTES={"foo": "bar"}
 RETAIN_INNER_QUOTES_AS_STRING='{"foo": "bar"}'
 RETAIN_INNER_QUOTES_AS_BACKTICKS=`{"foo": "bar's"}`
 TRIM_SPACE_FROM_UNQUOTED=    some spaced out string
-USERNAME=therealnerdybeast@example.tld
+EMAIL=therealnerdybeast@example.tld
     SPACED_KEY = parsed

test/parallel/test-dotenv-edge-cases.js

@@ -35,4 +35,17 @@ describe('.env supports edge cases', () => {
     assert.strictEqual(child.code, 0);
   });
 
+  it('should not override existing environment variables but introduce new vars', async () => {
+    const code = `
+      require('assert').strictEqual(process.env.BASIC, 'existing');
+      require('assert').strictEqual(process.env.AFTER_LINE, 'after_line');
+    `.trim();
+    const child = await common.spawnPromisified(
+      process.execPath,
+      [ `--env-file=${validEnvFilePath}`, '--eval', code ],
+      { cwd: __dirname, env: { ...process.env, BASIC: 'existing' } },
+    );
+    assert.strictEqual(child.stderr, '');
+    assert.strictEqual(child.code, 0);
+  });
 });

test/parallel/test-dotenv-node-options.js

@@ -48,10 +48,15 @@ describe('.env supports NODE_OPTIONS', () => {
     const code = `
       require('assert')(new Date().toString().includes('Hawaii'))
     `.trim();
+    // Some CI environments set TZ. Since an env file doesn't override existing
+    // environment variables, we need to delete it and then pass the env object
+    // as the environment to spawnPromisified.
+    const env = { ...process.env };
+    delete env.TZ;
     const child = await common.spawnPromisified(
       process.execPath,
       [ `--env-file=${relativePath}`, '--eval', code ],
-      { cwd: __dirname },
+      { cwd: __dirname, env },
     );
     assert.strictEqual(child.stderr, '');
     assert.strictEqual(child.code, 0);

test/parallel/test-dotenv.js

@@ -65,6 +65,6 @@ assert.strictEqual(process.env.RETAIN_INNER_QUOTES_AS_BACKTICKS, '{"foo": "bar\'
 // Retains spaces in string
 assert.strictEqual(process.env.TRIM_SPACE_FROM_UNQUOTED, 'some spaced out string');
 // Parses email addresses completely
-assert.strictEqual(process.env.USERNAME, 'therealnerdybeast@example.tld');
+assert.strictEqual(process.env.EMAIL, 'therealnerdybeast@example.tld');
 // Parses keys and values surrounded by spaces
 assert.strictEqual(process.env.SPACED_KEY, 'parsed');