Fix joining of source when source is an array of strings (#186)

* Fix joining of source when source is an array of strings

* Add test

Author: jtpio

javascript/src/ycell.ts

@@ -113,7 +113,7 @@ export const createCell = (
   }
   if (cell.source != null) {
     ycell.setSource(
-      typeof cell.source === 'string' ? cell.source : cell.source.join('\n')
+      typeof cell.source === 'string' ? cell.source : cell.source.join('')
     );
   }
   return ycell;

javascript/test/ycell.spec.ts

@@ -2,7 +2,7 @@
 // Copyright (c) Jupyter Development Team.
 // Distributed under the terms of the Modified BSD License.
 
-import { IMapChange, YCodeCell, YNotebook } from '../src';
+import { createStandaloneCell, IMapChange, YCodeCell, YNotebook } from '../src';
 
 describe('@jupyter/ydoc', () => {
   // Fix awareness timeout open handle
@@ -13,6 +13,17 @@ describe('@jupyter/ydoc', () => {
     jest.clearAllTimers();
   });
 
+  describe('createStandaloneCell', () => {
+    test('should convert a source set as a list of string to a single string', () => {
+      const cell = createStandaloneCell({
+        cell_type: 'code',
+        source: ['import this\n', 'import math']
+      });
+
+      expect(cell.source).toEqual('import this\nimport math');
+    });
+  });
+
   describe('YCell standalone', () => {
     test('should set source', () => {
       const codeCell = YCodeCell.create();