Throw an error when the dataLength prop is missing

Previously, missing the mandatory dataLength prop resulted in the component
silently failing to add more content. This commit makes the issue more visible,
as the component will now throw an error after mounting if the dataLength prop
isn't provided.

Author: Karl Johansson

src/__tests__/index.test.tsx

@@ -3,7 +3,12 @@ import { render, cleanup } from '@testing-library/react';
 import InfiniteScroll from '../index';
 
 describe('React Infinite Scroll Component', () => {
-  afterEach(cleanup);
+  const originalConsoleError = console.error;
+
+  afterEach(() => {
+    cleanup();
+    console.error = originalConsoleError;
+  });
 
   it('renders .infinite-scroll-component', () => {
     const { container } = render(
@@ -78,6 +83,18 @@ describe('React Infinite Scroll Component', () => {
     expect(onScrollMock).toHaveBeenCalled();
   });
 
+  describe('When missing the dataLength prop', () => {
+    it('throws an error', () => {
+      console.error = jest.fn();
+      const props = { loader: 'Loading...', hasMore: false, next: (() => {}) }
+
+      // @ts-ignore
+      expect(() => render(<InfiniteScroll {...props} />)).toThrow(Error)
+      // @ts-ignore
+      expect(console.error.mock.calls[0][0]).toContain('"dataLength" is missing')
+    });
+  })
+
   describe('When user scrolls to the bottom', () => {
     it('does not show loader if hasMore is false', () => {
       const { container, queryByText } = render(

src/index.tsx

@@ -66,6 +66,13 @@ export default class InfiniteScroll extends Component<Props, State> {
   private maxPullDownDistance = 0;
 
   componentDidMount() {
+    if (typeof this.props.dataLength === 'undefined') {
+      throw new Error(
+        `mandatory prop "dataLength" is missing. The prop is needed` +
+        ` when loading more content. Check README.md for usage`
+      )
+    }
+
     this._scrollableNode = this.getScrollableTarget();
     this.el = this.props.height
       ? this._infScroll