add test components

src/components/Hello.tsx

@@ -0,0 +1,28 @@
+import * as React from 'react';
+
+export interface Props {
+  name: string;
+  enthusiasmLevel?: number;
+}
+
+export default class Hello extends React.Component<Props, object> {
+  render() {
+    const { name, enthusiasmLevel = 1 } = this.props;
+
+    if (enthusiasmLevel <= 0) {
+      throw new Error('You could be a little more enthusiastic. :D');
+    }
+
+    return (
+      <div className="hello">
+        <div className="greeting">
+          Hello {name + getExclamationMarks(enthusiasmLevel)}
+        </div>
+      </div>
+    );
+  }
+}
+
+function getExclamationMarks(numChars: number) {
+  return Array(numChars + 1).join('!');
+}