Set NODE_ENV to to 'test' if not already set

lib/fork.js

@@ -10,12 +10,10 @@ if (fs.realpathSync(__filename) !== __filename) {
 	console.warn('WARNING: `npm link ava` and the `--preserve-symlink` flag are incompatible. We have detected that AVA is linked via `npm link`, and that you are using either an early version of Node 6, or the `--preserve-symlink` flag. This breaks AVA. You should upgrade to Node 6.2.0+, avoid the `--preserve-symlink` flag, or avoid using `npm link ava`.');
 }
 
-let env = process.env;
+const env = Object.assign({NODE_ENV: 'test'}, process.env);
 
 // Ensure NODE_PATH paths are absolute
 if (env.NODE_PATH) {
-	env = Object.assign({}, env);
-
 	env.NODE_PATH = env.NODE_PATH
 		.split(path.delimiter)
 		.map(x => path.resolve(x))

readme.md

@@ -1127,6 +1127,8 @@ t.true(a.test(b) || b === c)
 
 Each test file is run in a separate Node.js process. This allows you to change the global state or overriding a built-in in one test file, without affecting another. It's also great for performance on modern multi-core processors, allowing multiple test files to execute in parallel.
 
+AVA will set `process.env.NODE_ENV` to `test`, unless the `NODE_ENV` environment variable has been set. This is useful if the code you're testing has test defaults (for example when picking what database to connect to, or environment-specific Babel options). It may cause your code or its dependencies to behave differently though. Note that `'NODE_ENV' in process.env` will always be `true`.
+
 ## Tips
 
 ### Temp files

test/cli.js

@@ -802,3 +802,19 @@ test('--color enables formatting colors', t => {
 		t.end();
 	});
 });
+
+test('sets NODE_ENV to test when it is not set', t => {
+	execCli([path.join('fixture', 'node-env-test.js')], {env: {}}, (err, stdout, stderr) => {
+		t.ifError(err);
+		t.match(stderr, /1 passed/);
+		t.end();
+	});
+});
+
+test('doesn\'t set NODE_ENV when it is set', t => {
+	execCli([path.join('fixture', 'node-env-foo.js')], {env: {NODE_ENV: 'foo'}}, (err, stdout, stderr) => {
+		t.ifError(err);
+		t.match(stderr, /1 passed/);
+		t.end();
+	});
+});

test/fixture/node-env-foo.js

@@ -0,0 +1,6 @@
+import test from '../../';
+
+test('NODE_ENV is foo', t => {
+  t.plan(1);
+  t.is(process.env.NODE_ENV, 'foo');
+});

test/fixture/node-env-test.js

@@ -0,0 +1,6 @@
+import test from '../../';
+
+test('NODE_ENV is test', t => {
+  t.plan(1);
+  t.is(process.env.NODE_ENV, 'test');
+});