src: don't overwrite environment from .env file

src/node_dotenv.cc

@@ -48,14 +48,19 @@ void Dotenv::SetEnvironment(node::Environment* env) {
   for (const auto& entry : store_) {
     auto key = entry.first;
     auto value = entry.second;
-    env->env_vars()->Set(
-        isolate,
-        v8::String::NewFromUtf8(
-            isolate, key.data(), NewStringType::kNormal, key.size())
-            .ToLocalChecked(),
-        v8::String::NewFromUtf8(
-            isolate, value.data(), NewStringType::kNormal, value.size())
-            .ToLocalChecked());
+
+    auto existing = env->env_vars()->Get(key.data());
+
+    if (existing.IsNothing()) {
+      env->env_vars()->Set(
+          isolate,
+          v8::String::NewFromUtf8(
+              isolate, key.data(), NewStringType::kNormal, key.size())
+              .ToLocalChecked(),
+          v8::String::NewFromUtf8(
+              isolate, value.data(), NewStringType::kNormal, value.size())
+              .ToLocalChecked());
+    }
   }
 }
 

test/fixtures/dotenv/simple.env

@@ -0,0 +1,2 @@
+A=1
+B=2

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

@@ -6,6 +6,7 @@ const { describe, it } = require('node:test');
 
 const validEnvFilePath = '../fixtures/dotenv/valid.env';
 const relativePath = '../fixtures/dotenv/node-options.env';
+const simpleEnvFilePath = '../fixtures/dotenv/simple.env';
 
 describe('.env supports edge cases', () => {
 
@@ -35,4 +36,29 @@ describe('.env supports edge cases', () => {
     assert.strictEqual(child.code, 0);
   });
 
+  it('should not override existing environment variables', async () => {
+    const code = `
+      require('assert').strictEqual(process.env.BASIC, 'existing');
+    `.trim();
+    const child = await common.spawnPromisified(
+      process.execPath,
+      [ `--env-file=${validEnvFilePath}`, '--eval', code ],
+      { cwd: __dirname, env: { BASIC: 'existing' } },
+    );
+    assert.strictEqual(child.stderr, '');
+    assert.strictEqual(child.code, 0);
+  });
+
+  it('should not override existing environment variables but introduce new vars', async () => {
+    const code = `
+      require('assert').strictEqual(process.env.A + "," + process.env.B, '3,2');
+    `.trim();
+    const child = await common.spawnPromisified(
+      process.execPath,
+      [ `--env-file=${simpleEnvFilePath}`, '--eval', code ],
+      { cwd: __dirname, env: { A: '3' } },
+    );
+    assert.strictEqual(child.stderr, '');
+    assert.strictEqual(child.code, 0);
+  });
 });