refact(es6): use es6 syntax.

* fixed #10, But iojs-2 not support Object.assign, so use object-assign
  shim instead for now.
* use es6 syntax.
* use eslint to instead jshint.

index.js

@@ -14,43 +14,56 @@
  * Module dependencies.
  */
 
-var debug = require('debug')('koa-locales');
-var ini = require('ini');
-var util = require('util');
-var fs = require('fs');
-var path = require('path');
-var ms = require('humanize-ms');
-var merge = require('merge-descriptors');
+const debug = require('debug')('koa-locales');
+const ini = require('ini');
+const util = require('util');
+const fs = require('fs');
+const path = require('path');
+const ms = require('humanize-ms');
+const assign = require('object-assign');
+
+const DEFAULT_OPTIONS = {
+  defaultLocale: 'en-US',
+  queryField: 'locale',
+  cookieField: 'locale',
+  cookieMaxAge: '1y',
+  dir: undefined,
+  dirs: [path.join(process.cwd(), 'locales')],
+  functionName: '__',
+};
 
 module.exports = function (app, options) {
-  options = options || {};
-  var defaultLocale = formatLocale(options.defaultLocale || 'en-US');
-  var queryField = options.queryField || 'locale';
-  var cookieField = options.cookieField || 'locale';
-  var cookieMaxAge = ms(options.cookieMaxAge || '1y');
-  var localeDir = options.dir;
-  var localeDirs = options.dirs || [path.join(process.cwd(), 'locales')];
-  var functionName = options.functionName || '__';
-  var resources = {};
+  options = assign({}, DEFAULT_OPTIONS, options);
+  const defaultLocale = formatLocale(options.defaultLocale);
+  const queryField = options.queryField;
+  const cookieField = options.cookieField;
+  const cookieMaxAge = ms(options.cookieMaxAge);
+  const localeDir = options.dir;
+  const localeDirs = options.dirs;
+  const functionName = options.functionName;
+  const resources = {};
 
+  /**
+   * @Deprecated Use options.dirs instead.
+   */
   if (localeDir && localeDirs.indexOf(localeDir) === -1) {
     localeDirs.push(localeDir);
   }
 
-  for (var i = 0; i < localeDirs.length; i ++) {
-    var dir = localeDirs[i];
+  for (let i = 0; i < localeDirs.length; i++) {
+    const dir = localeDirs[i];
 
     if (!fs.existsSync(dir)) {
       continue;
     }
 
-    var names = fs.readdirSync(dir);
-    for (var j = 0; j < names.length; j++) {
-      var name = names[j];
-      var filepath = path.join(dir, name);
+    const names = fs.readdirSync(dir);
+    for (let j = 0; j < names.length; j++) {
+      const name = names[j];
+      const filepath = path.join(dir, name);
       // support en_US.js => en-US.js
-      var locale = formatLocale(name.split('.')[0]);
-      var resource = {};
+      const locale = formatLocale(name.split('.')[0]);
+      let resource = {};
 
       if (name.endsWith('.js') || name.endsWith('.json')) {
         resource = flattening(require(filepath));
@@ -59,46 +72,22 @@ module.exports = function (app, options) {
       }
 
       resources[locale] = resources[locale] || {};
-      merge(resources[locale], resource);
+      assign(resources[locale], resource);
     }
   }
 
   debug('init locales with %j, got %j resources', options, Object.keys(resources));
 
-  var ARRAY_INDEX_RE = /\{(\d+)\}/g;
-  function formatWithArray(text, values) {
-    return text.replace(ARRAY_INDEX_RE, function (orignal, matched) {
-      var index = parseInt(matched);
-      if (index < values.length) {
-        return values[index];
-      }
-      // not match index, return orignal text
-      return orignal;
-    });
-  }
-
-  var Object_INDEX_RE = /\{(.+?)\}/g;
-  function formatWithObject(text, values) {
-    return text.replace(Object_INDEX_RE, function (orignal, matched) {
-      var value = values[matched];
-      if (value) {
-        return value;
-      }
-      // not match index, return orignal text
-      return orignal;
-    });
-  }
-
   app.context[functionName] = function (key, value) {
     if (arguments.length === 0) {
       // __()
       return '';
     }
 
-    var locale = this.__getLocale();
-    var resource = resources[locale] || {};
+    const locale = this.__getLocale();
+    const resource = resources[locale] || {};
 
-    var text = resource[key] || key;
+    const text = resource[key] || key;
     debug('%s: %j => %j', locale, key, text);
     if (!text) {
       return '';
@@ -130,9 +119,9 @@ module.exports = function (app, options) {
     }
 
     // __(key, value1, ...)
-    var args = new Array(arguments.length);
+    const args = new Array(arguments.length);
     args[0] = text;
-    for(var i = 1; i < args.length; i++) {
+    for(let i = 1; i < args.length; i++) {
       args[i] = arguments[i];
     }
     return util.format.apply(util, args);
@@ -146,20 +135,20 @@ module.exports = function (app, options) {
       return this.__locale;
     }
 
-    var cookieLocale = this.cookies.get(cookieField);
-    var locale = this.query[queryField] || cookieLocale;
+    const cookieLocale = this.cookies.get(cookieField);
+    let locale = this.query[queryField] || cookieLocale;
     if (!locale) {
       // Accept-Language: zh-CN,zh;q=0.5
       // Accept-Language: zh-CN
-      var languages = this.acceptsLanguages();
+      let languages = this.acceptsLanguages();
       if (languages) {
         if (Array.isArray(languages)) {
           if (languages[0] === '*') {
             languages = languages.slice(1);
           }
           if (languages.length > 0) {
-            for (var i = 0; i < languages.length; i++) {
-              var lang = formatLocale(languages[i]);
+            for (let i = 0; i < languages.length; i++) {
+              const lang = formatLocale(languages[i]);
               if (resources[lang]) {
                 locale = lang;
                 break;
@@ -199,25 +188,49 @@ module.exports = function (app, options) {
     this.__locale = locale;
     return locale;
   };
-
-  function formatLocale(locale) {
-    // support zh_CN, en_US => zh-CN, en-US
-    return locale.replace('_', '-').toLowerCase();
-  }
 };
 
 function isObject(obj) {
   return Object.prototype.toString.call(obj) === '[object Object]';
 }
 
+const ARRAY_INDEX_RE = /\{(\d+)\}/g;
+function formatWithArray(text, values) {
+  return text.replace(ARRAY_INDEX_RE, function (orignal, matched) {
+    const index = parseInt(matched);
+    if (index < values.length) {
+      return values[index];
+    }
+    // not match index, return orignal text
+    return orignal;
+  });
+}
+
+const Object_INDEX_RE = /\{(.+?)\}/g;
+function formatWithObject(text, values) {
+  return text.replace(Object_INDEX_RE, function (orignal, matched) {
+    const value = values[matched];
+    if (value) {
+      return value;
+    }
+    // not match index, return orignal text
+    return orignal;
+  });
+}
+
+function formatLocale(locale) {
+  // support zh_CN, en_US => zh-CN, en-US
+  return locale.replace('_', '-').toLowerCase();
+}
+
 function flattening(data) {
 
-  var result = {};
+  const result = {};
 
   function deepFlat (data, keys) {
     Object.keys(data).forEach(function(key) {
-      var value = data[key];
-      var k = keys ? keys + '.' + key : key;
+      const value = data[key];
+      const k = keys ? keys + '.' + key : key;
       if (isObject(value)) {
         deepFlat(value, k);
       } else {