Add unit test for DataFrame.length === 0

src/es6/__tests__/core/frame.js

@@ -1014,4 +1014,19 @@ describe('frame', () => {
       expect(df2.get('q').name).toEqual('q');
     });
   });
+
+  describe('length', () => {
+    it('Has length zero when empty DataFrame', () => {
+      const df = new DataFrame();
+      expect(df.length).toEqual(0);
+
+      const df2 = new DataFrame([]);
+      expect(df2.length).toEqual(0);
+    });
+
+    it('Estimates non-zero length properly', () => {
+      const df = new DataFrame(Immutable.Map({x: new Series([1, 2, 5, 4, 3])}));
+      expect(df.length).toEqual(5);
+    });
+  });
 });

src/es6/core/frame.js

@@ -327,9 +327,7 @@ export default class DataFrame extends NDFrame {
    * df.length;
    */
   get length(): number {
-    if (this._data.keySeq().size === 0)
-      return 0;
-    return Math.max(...this._data.keySeq().map(k => this.get(k).length).toArray());
+    return Math.max(0, ...this.columns.map(k => this.get(k).length).toArray());
   }
 
   /**