[Fix] shallow: simulate: ensure it returns itself.

Also make mount simulate tests the same as shallow's.

Fixes #1601

Author: ljharb

packages/enzyme-test-suite/test/ReactWrapper-spec.jsx

@@ -1500,6 +1500,21 @@ describeWithDOM('mount', () => {
         .to.throw(TypeError, "ReactWrapper::simulate() event 'invalidEvent' does not exist");
     });
 
+    describeIf(!REACT013, 'stateless function component', () => {
+      it('should pass in event data', () => {
+        const spy = sinon.spy();
+        const Foo = () => (
+          <a onClick={spy}>foo</a>
+        );
+
+        const wrapper = mount(<Foo />);
+
+        wrapper.simulate('click', { someSpecialData: 'foo' });
+        expect(spy.calledOnce).to.equal(true);
+        expect(spy.args[0][0].someSpecialData).to.equal('foo');
+      });
+    });
+
     describe('Normalizing JS event names', () => {
       it('should convert lowercase events to React camelcase', () => {
         const spy = sinon.spy();
@@ -1539,19 +1554,37 @@ describeWithDOM('mount', () => {
       });
     });
 
-    describeIf(!REACT013, 'stateless function component', () => {
-      it('should pass in event data', () => {
-        const spy = sinon.spy();
-        const Foo = () => (
-          <a onClick={spy}>foo</a>
-        );
+    it('should be batched updates', () => {
+      let renderCount = 0;
+      class Foo extends React.Component {
+        constructor(props) {
+          super(props);
+          this.state = {
+            count: 0,
+          };
+          this.onClick = this.onClick.bind(this);
+        }
+        onClick() {
+          this.setState({ count: this.state.count + 1 });
+          this.setState({ count: this.state.count + 1 });
+        }
+        render() {
+          renderCount += 1;
+          return (
+            <a onClick={this.onClick}>{this.state.count}</a>
+          );
+        }
+      }
 
-        const wrapper = mount(<Foo />);
+      const wrapper = mount(<Foo />);
+      wrapper.simulate('click');
+      expect(wrapper.text()).to.equal('1');
+      expect(renderCount).to.equal(2);
+    });
 
-        wrapper.simulate('click', { someSpecialData: 'foo' });
-        expect(spy.calledOnce).to.equal(true);
-        expect(spy.args[0][0].someSpecialData).to.equal('foo');
-      });
+    it('chains', () => {
+      const wrapper = mount(<div />);
+      expect(wrapper.simulate('click')).to.equal(wrapper);
     });
   });
 

packages/enzyme-test-suite/test/ShallowWrapper-spec.jsx

@@ -1386,6 +1386,11 @@ describe('shallow', () => {
       expect(wrapper.text()).to.equal('1');
       expect(renderCount).to.equal(2);
     });
+
+    it('chains', () => {
+      const wrapper = shallow(<div />);
+      expect(wrapper.simulate('click')).to.equal(wrapper);
+    });
   });
 
   describe('.setState(newState[, callback])', () => {

packages/enzyme/src/ReactWrapper.js

@@ -573,11 +573,11 @@ class ReactWrapper {
    * @returns {ReactWrapper}
    */
   simulate(event, mock = {}) {
-    this.single('simulate', (n) => {
+    return this.single('simulate', (n) => {
       this[RENDERER].simulateEvent(n, event, mock);
       this[ROOT].update();
+      return this;
     });
-    return this;
   }
 
   /**

packages/enzyme/src/ShallowWrapper.js

@@ -709,6 +709,7 @@ class ShallowWrapper {
     return this.single('simulate', (n) => {
       this[RENDERER].simulateEvent(n, event, ...args);
       this[ROOT].update();
+      return this;
     });
   }