diff --git a/__tests__/async-syntax-highlighter.js b/__tests__/async-syntax-highlighter.js
new file mode 100644
index 000000000..527677a00
--- /dev/null
+++ b/__tests__/async-syntax-highlighter.js
@@ -0,0 +1,89 @@
+import React from 'react';
+import renderer from 'react-test-renderer';
+import AsyncSyntaxHighlighter from "../src/async-syntax-highlighter";
+
+test('AsyncSyntaxHighlighter registerLanguage when registerLanguage is undefined', () => {
+  const SyntaxHighlighter = AsyncSyntaxHighlighter({});
+
+  SyntaxHighlighter.registerLanguage("test", {});
+  expect(SyntaxHighlighter.languages).toEqual([]);
+});
+
+test('AsyncSyntaxHighlighter registerLanguage when registerLanguage is defined but astGenerator isn\'t', () => {
+  const registerLanguage = jest.fn();
+  const SyntaxHighlighter = AsyncSyntaxHighlighter({ registerLanguage });
+
+  SyntaxHighlighter.registerLanguage("test", {});
+  expect(SyntaxHighlighter.languages).toEqual([ { name: "test", language: {} }]);
+});
+
+test('AsyncSyntaxHighlighter registerLanguage when registerLanguage & astGenerator is defined', () => {
+  const registerLanguage = jest.fn();
+  
+  const SyntaxHighlighter = AsyncSyntaxHighlighter({ registerLanguage });
+  SyntaxHighlighter.astGenerator = { test: '123' };
+
+  SyntaxHighlighter.registerLanguage("test", {});
+  expect(SyntaxHighlighter.languages).not.toEqual([ { name: "test", language: {} }]);
+  expect(registerLanguage).toBeCalledWith(SyntaxHighlighter.astGenerator, "test", {})
+});
+
+test('AsyncSyntaxHighlighter isRegistered when registerLanguage is not defined', () => {
+  const SyntaxHighlighter = AsyncSyntaxHighlighter({ });
+
+  expect(SyntaxHighlighter.isRegistered("test")).toEqual(true);
+});
+
+test('AsyncSyntaxHighlighter isRegistered when astGenerator is not defined it checks the languages array', () => {
+  const registerLanguage = jest.fn();
+  
+  const SyntaxHighlighter = AsyncSyntaxHighlighter({ registerLanguage });
+  SyntaxHighlighter.astGenerator = null;
+  SyntaxHighlighter.languages.push({ name: 'test' })
+  expect(SyntaxHighlighter.isRegistered('test')).toEqual(true);
+});
+
+test('AsyncSyntaxHighlighter isRegistered when astGenerator is defined it should call isLanguageRegistered', () => {
+  const registerLanguage = jest.fn();
+  
+  const SyntaxHighlighter = AsyncSyntaxHighlighter({ registerLanguage });
+  SyntaxHighlighter.astGenerator = null;
+  SyntaxHighlighter.languages.push({ name: 'test' })
+  expect(SyntaxHighlighter.isRegistered('test')).toEqual(true);
+});
+
+test('AsyncSyntaxHighlighter loadAstGenerator should return the promise of the loader', () => {
+  const testValue = "test";
+  const loader = jest.fn().mockReturnValue({
+    then: jest.fn().mockReturnValue(testValue)
+  })
+  
+  
+  const SyntaxHighlighter = AsyncSyntaxHighlighter({ loader });
+  expect(SyntaxHighlighter.loadAstGenerator()).toEqual(testValue);
+});
+
+test('AsyncSyntaxHighlighter loadAstGenerator when astGenerator resolves', async () => {
+  const astGenerator = "test";
+  const loader = jest.fn().mockResolvedValue(astGenerator)
+  
+  
+  const SyntaxHighlighter = AsyncSyntaxHighlighter({ loader });
+  await SyntaxHighlighter.loadAstGenerator();
+  expect(SyntaxHighlighter.astGenerator).toEqual(astGenerator)
+});
+
+test('AsyncSyntaxHighlighter loadAstGenerator when astGenerator resolves and it has languages in the language array', async () => {
+  const astGenerator = "test";
+  const registerLanguage = jest.fn();
+  const loader = jest.fn().mockResolvedValue(astGenerator)
+  
+  
+  const SyntaxHighlighter = AsyncSyntaxHighlighter({ loader, registerLanguage });
+  const testLanguage = { name: "cpp", language: { } };
+
+  SyntaxHighlighter.languages.push(testLanguage);
+
+  await SyntaxHighlighter.loadAstGenerator();
+  expect(registerLanguage).toBeCalledWith(astGenerator, testLanguage.name, testLanguage.language)
+});
\ No newline at end of file
diff --git a/__tests__/prism-async-light.js b/__tests__/prism-async-light.js
index 0e9c695c6..8ecebc8ce 100644
--- a/__tests__/prism-async-light.js
+++ b/__tests__/prism-async-light.js
@@ -3,8 +3,6 @@ import renderer from 'react-test-renderer';
 import { PrismAsyncLight as SyntaxHighlighter } from "../src";
 import prism from "../src/styles/prism/prism";
 
-
-
 test('SyntaxHighlighter renders jsx highlighted text', () => {
   const tree = renderer.create(
    <SyntaxHighlighter language="jsx" style={prism}> 
@@ -47,7 +45,6 @@ test('SyntaxHighlighter should just render text if syntax is not registered', ()
 });
 
 test('When the code split is loaded - SyntaxHighlighter renders jsx highlighted text', async () => {
-    
     await SyntaxHighlighter.preload();
       
     const tree = renderer.create(
@@ -77,4 +74,4 @@ test('When the code split is loaded - SyntaxHighlighter renders jsx highlighted
     ).toJSON();
     
     expect(tree).toMatchSnapshot();
-});
\ No newline at end of file
+});
diff --git a/src/async-syntax-highlighter.js b/src/async-syntax-highlighter.js
index 3209342c2..d78dc78e1 100644
--- a/src/async-syntax-highlighter.js
+++ b/src/async-syntax-highlighter.js
@@ -24,7 +24,7 @@ export default (options) => {
 
       if (!ReactAsyncHighlighter.astGenerator) {
         // Ast generator not available yet, but language will be registered once it is.
-        return ReactAsyncHighlighter.languages.includes(item => item.name === language);
+        return ReactAsyncHighlighter.languages.findIndex(item => item.name === language) > -1;
       }
 
       return isLanguageRegistered(ReactAsyncHighlighter.astGenerator, language);
@@ -44,7 +44,6 @@ export default (options) => {
         });
       }
     };
-    
 
     static loadAstGenerator() {
       ReactAsyncHighlighter.astGeneratorPromise = loader().then(astGenerator => {