diff --git a/README.md b/README.md
index eb33720..08b2345 100644
--- a/README.md
+++ b/README.md
@@ -86,7 +86,7 @@ WordPOS.defaults = {
    * include data files in preload
    * @type {boolean}
    */
-  includeData: false,  // WIP
+  includeData: false,
 
   /**
    * set to true to enable debug logging
@@ -316,10 +316,6 @@ For CLI usage and examples, see [bin/README](bin).
 See [bench/README](bench).
 
 
-## TODO
-- implement `includeData` option for preload
-
-
 ## Changes
 See [CHANGELOG](./CHANGELOG.md).
 
diff --git a/package.json b/package.json
index 43e8db1..0a55726 100755
--- a/package.json
+++ b/package.json
@@ -1,6 +1,6 @@
 {
   "name": "wordpos",
-  "version": "2.0.0-beta.11",
+  "version": "2.0.0-beta.13",
   "description": "wordpos is a set of part-of-speech utilities for Node.js & browser using the WordNet database.",
   "author": "Moos <mooster@42at.com>",
   "keywords": [
@@ -69,8 +69,8 @@
     "prepare": "npm run build",
     "pretest": "node tools/makeJsonDict.js index data",
     "test": "npm run test-node && npm run test-browser",
-    "test-node": "mocha test",
-    "test-browser": "mocha test/wordpos_test --require @babel/register"
+    "test-node": "mocha test --exclude test/browser_test.js",
+    "test-browser": "mocha test/browser_test test/wordpos_test  --require @babel/register"
   },
   "license": "MIT"
 }
diff --git a/src/browser/baseFile.js b/src/browser/baseFile.js
index abce7c6..889e3b5 100644
--- a/src/browser/baseFile.js
+++ b/src/browser/baseFile.js
@@ -44,7 +44,8 @@ class BaseFile {
     this.options.debug && console.timeEnd('index load ' + this.posName)
     return promise
       .then(exports => {
-        this.file = exports.default
+        this.file = exports.default;
+        return this;
       })
       .catch(err => {
         console.error(`Error loading "${this.type}" file ${this.filePath}.`, err);
@@ -54,7 +55,7 @@ class BaseFile {
   }
 
   ready(fn, args) {
-    return this.load().then(() => fn.apply(this, args));
+    return this.load().then(res => fn && fn.apply(this, args) || res);
   }
 }
 
diff --git a/src/browser/index.js b/src/browser/index.js
index 131a539..c2f6cdc 100644
--- a/src/browser/index.js
+++ b/src/browser/index.js
@@ -34,6 +34,10 @@ class WordPOS {
     }
   }
 
+  ready() {
+    return this.loaded || Promise.resolve();
+  }
+
   initFiles() {
     const keys = Object.keys(POS);
     const loadOne = (Comp, pos) => new Comp(this.options.dictPath, POS[pos], this.options);
@@ -44,7 +48,7 @@ class WordPOS {
     this.dataFiles = reducer(loader(DataFile));
 
     if (this.options.preload) {
-      this.loaded = this.preloadIndexes(this.options.preload);
+      this.loaded = this.preloadFiles(this.options.preload);
     }
   }
 
@@ -61,23 +65,28 @@ class WordPOS {
    * @param  {string|Array} [pos] POS to load (default: all)
    * @return {Promise.<index data>}
    */
-  preloadIndexes(pos) {
-    let file = this.indexFile[pos];
-    let load = p => file.load();
+  preloadFiles(pos) {
+    let promise = this._preload(this.indexFiles, pos);
+    if (this.options.includeData) {
+      promise = Promise.all([].concat(promise, this._preload(this.dataFiles, pos)))
+        .then(res => res.flat());
+    }
+    return promise;
+  }
+
+  _preload(files, pos) {
+    let load = p => files[p].load();
     let promise;
 
     if (!pos || pos === true) { // preload all
       promise = Promise.all(Object.keys(POS).map(load));
     }
-    else if (typeof pos === 'string' && file) {
+    else if (typeof pos === 'string' && files[pos]) {
       promise = load(pos);
     }
     else if (pos instanceof Array) {
-      promise = pos.forEach(pos => file && load(pos));
+      promise = Promise.all(pos.map(load));
     }
-
-    // TODO includeData
-
     return promise || Promise.reject(new RangeError(`Unknown POS "${pos}" for preload.`));
   }
 
@@ -183,6 +192,8 @@ WordPOS.defaults = {
  */
 WordPOS.stopwords = stopwords;
 
+WordPOS.POS = POS;
+
 // Export as CJS handled by Parcel, otherwise will get WordPOS.default
 // if use: export default WordPOS;
 module.exports = WordPOS;
diff --git a/test/browser_test.js b/test/browser_test.js
new file mode 100644
index 0000000..d9ba8d7
--- /dev/null
+++ b/test/browser_test.js
@@ -0,0 +1,237 @@
+/**
+ * browser_test.js
+ *
+ *   test file for browser-specific  functionality
+ *
+ * Usage:
+ *   npm install mocha -g
+ *   mocha browser_test.js -require @babel/register
+ *
+ *   or
+ *
+ *   npm test
+ *
+ * Copyright (c) 2012-2020 mooster@42at.com
+ * https://github.com/moos/wordpos
+ *
+ * Released under MIT license
+ */
+
+ // used in src code to signal test mode
+global.window = global.window || {};
+global.window.__mocha = true;
+
+var
+  chai = require('chai'),
+  _ = require('underscore'),
+  assert = chai.assert,
+  browser = process.browser = process.argv.includes('@babel/register'),
+  WordPOS = require('../src/wordpos'),
+  wordpos,
+  path = require('path'),
+  dictPath = browser ? path.resolve('./test/dict') : undefined;
+
+const dictRequired = () => Object.keys(require.cache).filter(p => /\bdict\b/i.test(p));
+
+if (!browser) {
+  throw new Error('Not in browser mode!');
+}
+chai.config.showDiff = true;
+
+describe('options: preload', () => {
+
+  // clear require.cache before each test
+  beforeEach(() => {
+    dictRequired().forEach((m) => delete require.cache[m]);
+  });
+
+  it('preload: false', () => {
+    wordpos = new WordPOS({
+      preload: false,
+      dictPath: dictPath
+    });
+    return wordpos.ready().then(res => {
+      assert.equal(res, undefined);
+      assert.equal(dictRequired().length, 0);
+    });
+  });
+
+  it('preload: true', () => {
+    assert.equal(dictRequired().length, 0);
+    wordpos = new WordPOS({
+      preload: true,
+      dictPath: dictPath
+    });
+    return wordpos.ready().then(res => {
+      assert.equal(res.length, 4);
+      res.forEach(index => assert.equal(index.type, 'index'));
+      res.forEach(index => assert.equal(/\bdict.index\./.test(index.filePath), true));
+
+      let reqs = dictRequired();
+      assert.equal(reqs.length, 4);
+      reqs.forEach(req => assert.equal(/\bdict.index\./.test(req), true));
+      Object.values(WordPOS.POS).forEach(pos => assert.notEqual(reqs.join().indexOf(`index.${pos}.js`), -1));
+    });
+  });
+
+  it('preload: "r"', () => {
+    wordpos = new WordPOS({
+      preload: 'r',
+      dictPath: dictPath
+    });
+    return wordpos.ready().then(res => {
+      assert.equal(res.type, 'index');
+      assert.equal(res.posName, 'adv');
+      let reqs = dictRequired();
+      assert.equal(reqs.length, 1);
+      assert.equal(/index\.adv\.js/.test(reqs[0]), true);
+    });
+  });
+
+  it('preload: ["r","a"]', () => {
+    wordpos = new WordPOS({
+      preload: ['r','a'],
+      dictPath: dictPath
+    });
+    return wordpos.ready().then(res => {
+      assert.equal(res.length, 2);
+
+      // TODO -- order may NOT be always the same!!!
+      assert.equal(res[0].type, 'index');
+      assert.equal(res[0].posName, 'adv');
+      assert.equal(res[1].type, 'index');
+      assert.equal(res[1].posName, 'adj');
+
+      let reqs = dictRequired();
+      assert.equal(reqs.length, 2);
+      assert.equal(/index\.adv\.js/.test(reqs[0]), true);
+      assert.equal(/index\.adj\.js/.test(reqs[1]), true);
+    });
+  });
+
+  it('preload: "foo"', () => {
+    wordpos = new WordPOS({
+      preload: 'foo',
+      dictPath: dictPath
+    });
+    return wordpos.ready().then(res => {
+      // shouldn't get here
+      assert(false);
+    }).catch(err => {
+      assert.equal(err, 'RangeError: Unknown POS "foo" for preload.')
+    });
+  });
+});
+
+
+
+describe('options: preload with includeData', () => {
+
+  // clear require.cache before each test
+  beforeEach(() => {
+    dictRequired().forEach((m) => delete require.cache[m]);
+  });
+
+  it('preload: false', () => {
+    wordpos = new WordPOS({
+      preload: false,
+      includeData: true,
+      dictPath: dictPath
+    });
+    return wordpos.ready().then(res => {
+      assert.equal(res, undefined);
+      assert.equal(dictRequired().length, 0);
+    });
+  });
+
+  it('preload: true', () => {
+    assert.equal(dictRequired().length, 0);
+    wordpos = new WordPOS({
+      preload: true,
+      includeData: true,
+      dictPath: dictPath
+    });
+    return wordpos.ready().then(res => {
+      assert.equal(res.length, 8);
+      assert.equal(res.filter(m => m.type === 'index').length, 4);
+      assert.equal(res.filter(m => m.type === 'data').length, 4);
+      assert.equal(res.filter(m => /\bdict.index\./.test(m.filePath)).length, 4);
+      assert.equal(res.filter(m => /\bdict.data\./.test(m.filePath)).length, 4);
+
+      let reqs = dictRequired();
+      assert.equal(reqs.length, 8);
+      assert.equal(reqs.filter(m => /\bdict.index\./.test(m)).length, 4);
+      assert.equal(reqs.filter(m => /\bdict.data\./.test(m)).length, 4);
+
+      let reqsStr = reqs.join();
+      Object.values(WordPOS.POS).forEach(pos => {
+        assert.equal(reqs.filter(m => m.indexOf(`index.${pos}.js`) !== -1).length, 1);
+        assert.equal(reqs.filter(m => m.indexOf(`data.${pos}.js`) !== -1).length, 1);
+      });
+    });
+  });
+
+  it('preload: "r"', () => {
+    wordpos = new WordPOS({
+      preload: 'r',
+      includeData: true,
+      dictPath: dictPath
+    });
+    return wordpos.ready().then(res => {
+      assert.equal(res.length, 2);
+      assert.equal(res[0].type, 'index');
+      assert.equal(res[0].posName, 'adv');
+      assert.equal(res[1].type, 'data');
+      assert.equal(res[1].posName, 'adv');
+      let reqs = dictRequired();
+      assert.equal(reqs.length, 2);
+      assert.equal(/index\.adv\.js/.test(reqs[0]), true);
+      assert.equal(/data\.adv\.js/.test(reqs[1]), true);
+    });
+  });
+
+  it('preload: ["r","a"]', () => {
+    wordpos = new WordPOS({
+      preload: ['r','a'],
+      includeData: true,
+      dictPath: dictPath
+    });
+    return wordpos.ready().then(res => {
+      assert.equal(res.length, 4);
+
+      // TODO -- order may NOT be always the same!!!
+      assert.equal(res[0].type, 'index');
+      assert.equal(res[0].posName, 'adv');
+      assert.equal(res[1].type, 'index');
+      assert.equal(res[1].posName, 'adj');
+
+      assert.equal(res[2].type, 'data');
+      assert.equal(res[2].posName, 'adv');
+      assert.equal(res[3].type, 'data');
+      assert.equal(res[3].posName, 'adj');
+
+      let reqs = dictRequired();
+      assert.equal(reqs.length, 4);
+      assert.equal(/index\.adv\.js/.test(reqs[0]), true);
+      assert.equal(/index\.adj\.js/.test(reqs[1]), true);
+      assert.equal(/data\.adv\.js/.test(reqs[2]), true);
+      assert.equal(/data\.adj\.js/.test(reqs[3]), true);
+    });
+  });
+
+  it('preload: "foo"', () => {
+    wordpos = new WordPOS({
+      preload: 'foo',
+      includeData: true,
+      dictPath: dictPath
+    });
+    return wordpos.ready().then(res => {
+      // shouldn't get here
+      assert(false);
+    }).catch(err => {
+      assert.equal(err, 'RangeError: Unknown POS "foo" for preload.')
+    });
+  });
+
+
+});