feat(dotenv-flow): return error if no .env* files were found

With this change dotenv-flow's `.config()` method will start returning an error if there is no appropriate ".env*" files were found.

Closes #41.

lib/dotenv-flow.js

@@ -1,7 +1,7 @@
 'use strict';
 
 const fs = require('fs');
-const {resolve} = require('path');
+const {resolve, sep} = require('path');
 const dotenv = require('dotenv');
 
 /**
@@ -162,6 +162,14 @@ function config(options = {}) {
         .filter(filename => fs.existsSync(filename))
     );
 
+    if (existingFiles.length === 0) {
+      const message = node_env
+          ? `no ".env*" files matching pattern "${path + sep}.env[.${node_env}][.local]"`
+          : `no ".env*" files matching pattern "${path + sep}.env[.local]" (no \`process.env.NODE_ENV\` or \`options.default_node_env\` is defined)`;
+
+      return { error: new Error(message) };
+    }
+
     return load(existingFiles, { encoding, silent });
   }
   catch (error) {

test/unit/dotenv-flow-api.spec.js

@@ -770,7 +770,7 @@ describe('dotenv-flow (API)', () => {
       });
     });
 
-    describe('the return object', () => {
+    describe('the returning object', () => {
       it('includes the parsed contents of the files in the `parsed` property', () => {
         $dotenvFiles['/path/to/project/.env'] = 'DEFAULT_ENV_VAR=ok';
         $dotenvFiles['/path/to/project/.env.local'] = 'LOCAL_ENV_VAR=ok';
@@ -818,5 +818,84 @@ describe('dotenv-flow (API)', () => {
           .with.property('message', 'file reading error stub');
       });
     });
+
+    describe('when none of the appropriate ".env*" files is present', () => {
+      it('returns "no `.env*` files" error', () => {
+        const result = dotenvFlow.config();
+
+        expect(result)
+          .to.be.an('object')
+          .with.property('error')
+          .that.is.an('error')
+          .with.property('message')
+          .that.matches(/no "\.env\*" files/);
+      });
+
+      describe('and no `node_env`-specific environment is set', () => {
+        it('returns an error with a message indicating the working directory', () => {
+          const noopResult = dotenvFlow.config();
+
+          expect(noopResult.error)
+              .to.be.an('error')
+              .with.property('message')
+              .that.includes('/path/to/project');
+
+          const pathResult = dotenvFlow.config({
+            path: '/path/to/another/project'
+          });
+
+          expect(pathResult.error)
+              .to.be.an('error')
+              .with.property('message')
+              .that.includes('/path/to/another/project');
+        });
+
+        it('returns an error with a message indicating the default ".env[.local]" setup', () => {
+          const result = dotenvFlow.config();
+
+          const expectedPattern = '/path/to/project/.env[.local]';
+
+          expect(result.error)
+            .to.be.an('error')
+            .with.property('message')
+            .that.includes(`"${expectedPattern}"`);
+        });
+      });
+
+      describe('and `process.env.NODE_ENV` is specified', () => {
+        beforeEach('setup `process.env.NODE_ENV`', () => {
+          process.env.NODE_ENV = 'development';
+        });
+
+        it('returns an error with a message indicating the working directory', () => {
+          const noopResult = dotenvFlow.config();
+
+          expect(noopResult.error)
+            .to.be.an('error')
+            .with.property('message')
+            .that.includes('/path/to/project');
+
+          const pathResult = dotenvFlow.config({
+            path: '/path/to/another/project'
+          });
+
+          expect(pathResult.error)
+            .to.be.an('error')
+            .with.property('message')
+            .that.includes('/path/to/another/project');
+        });
+
+        it('returns an error with a message indicating the node_env-specific setup', () => {
+          const result = dotenvFlow.config();
+
+          const expectedPattern = '/path/to/project/.env[.development][.local]';
+
+          expect(result.error)
+            .to.be.an('error')
+            .with.property('message')
+            .that.includes(`"${expectedPattern}"`);
+        });
+      });
+    });
   });
 });