Throw an error if the first argument for the parser is not a string

This is to give the user gets feedback if the first argument is
invalid. Also, add tests to confirm behavior.

Author: remarkablemark

index.js

@@ -51,6 +51,9 @@
      * @return {ReactElement|Array}
      */
     function HTMLReactParser(html, options) {
+        if (typeof html !== 'string') {
+            throw new Error('`HTMLReactParser`: The first argument must be a string.');
+        }
         return domToReact(htmlToDOM(html), options);
     }
 

test/html-to-react.js

@@ -29,6 +29,18 @@ describe('html-to-react', function() {
      */
     describe('parser', function() {
 
+        it('throws an error if first argument is not a string', function() {
+            assert.throws(function() { Parser(); });
+
+            [undefined, null, {}, [], 42].forEach(function(arg) {
+                assert.throws(function() { Parser(arg); });
+            });
+        });
+
+        it('returns string if cannot be parsed to HTML', function() {
+            assert.equal(Parser('foo'), 'foo');
+        });
+
         it('converts single HTML element to React', function() {
             var html = data.html.single;
             var reactElement = Parser(html);