change the 'exists' method to check optional instead of 'resolve'

src/Translator.js

@@ -2,6 +2,7 @@ import baseLogger from './logger.js';
 import EventEmitter from './EventEmitter.js';
 import postProcessor from './postProcessor.js';
 import * as utils from './utils.js';
+import { isOptional } from './utils.js';
 
 const checkedLoadedFor = {};
 
@@ -39,6 +40,10 @@ class Translator extends EventEmitter {
   }
 
   exists(key, options = { interpolation: {} }) {
+    if (isOptional(key)) {
+      return false;
+    }
+
     const resolved = this.resolve(key, options);
     return resolved && resolved.res !== undefined;
   }
@@ -84,7 +89,7 @@ class Translator extends EventEmitter {
     if (!options) options = {};
 
     // non valid keys handling
-    if (keys === undefined || keys === null /* || keys === ''*/) return '';
+    if (isOptional(keys) /* || keys === ''*/) return '';
     if (!Array.isArray(keys)) keys = [String(keys)];
 
     // separators
@@ -364,8 +369,7 @@ class Translator extends EventEmitter {
     let usedLng;
     let usedNS;
 
-    if (!keys) keys = [];
-    else if (typeof keys === 'string') keys = [keys];
+    if (typeof keys === 'string') keys = [keys];
 
     // forEach possible key
     keys.forEach(k => {

src/utils.js

@@ -139,3 +139,7 @@ export const isIE10 =
   window.navigator &&
   window.navigator.userAgent &&
   window.navigator.userAgent.indexOf('MSIE') > -1;
+
+export function isOptional(str) {
+  return str === undefined || str === null;
+}