setState now creates new versions instead of mutating state

src/component.js

@@ -46,14 +46,16 @@ extend(Component.prototype, {
 
 	/**
 	 * Update component state and schedule a re-render.
-	 * @param {object} state A hash of state properties to update with new values
+	 * @param {object} state A dict of state properties to be shallowly merged
+	 * 	into the current state, or a function that will produce such a dict. The
+	 * 	function is called with the current state and props.
 	 * @param {() => void} callback A function to be called once component state is
 	 * 	updated
 	 */
 	setState(state, callback) {
-		let s = this.state;
-		if (!this.prevState) this.prevState = extend({}, s);
-		extend(s, typeof state==='function' ? state(s, this.props) : state);
+		const prev = this.prevState = this.state;
+		if (typeof state === 'function') state = state(prev, this.props);
+		this.state = extend(extend({}, prev), state);
 		if (callback) this._renderCallbacks.push(callback);
 		enqueueRender(this);
 	},

src/vdom/component.js

@@ -84,8 +84,8 @@ export function renderComponent(component, renderMode, mountAll, isChild) {
 		rendered, inst, cbase;
 
 	if (component.constructor.getDerivedStateFromProps) {
-		previousState = extend({}, previousState);
-		component.state = extend(state, component.constructor.getDerivedStateFromProps(props, state));
+		state = extend(extend({}, state), component.constructor.getDerivedStateFromProps(props, state));
+		component.state = state;
 	}
 
 	// if updating

test/browser/lifecycle.js

@@ -505,6 +505,31 @@ describe('Lifecycle methods', () => {
 				value: 3
 			});
 		});
+
+		it('should NOT mutate state, only create new versions', () => {
+			const stateConstant = {};
+			let componentState;
+
+			class Stateful extends Component {
+				static getDerivedStateFromProps() {
+					return {key: 'value'};
+				}
+
+				constructor() {
+					super(...arguments);
+					this.state = stateConstant;
+				}
+
+				componentDidMount() {
+					componentState = this.state;
+				}
+			}
+
+			render(<Stateful />, scratch);
+
+			expect(componentState).to.deep.equal({key: 'value'});
+			expect(stateConstant).to.deep.equal({});
+		});
 	});
 
 	describe("#getSnapshotBeforeUpdate", () => {
@@ -1186,7 +1211,7 @@ describe('Lifecycle methods', () => {
 	});
 
 
-	describe('shouldComponentUpdate', () => {
+	describe('#shouldComponentUpdate', () => {
 		let setState;
 
 		class Should extends Component {
@@ -1325,6 +1350,36 @@ describe('Lifecycle methods', () => {
 	});
 
 
+	describe('#setState', () => {
+		it('should NOT mutate state, only create new versions', () => {
+			const stateConstant = {};
+			let didMount = false;
+			let componentState;
+
+			class Stateful extends Component {
+				constructor() {
+					super(...arguments);
+					this.state = stateConstant;
+				}
+
+				componentDidMount() {
+					didMount = true;
+					this.setState({key: 'value'}, () => {
+						componentState = this.state;
+					});
+				}
+			}
+
+			render(<Stateful />, scratch);
+			rerender();
+
+			expect(didMount).to.equal(true);
+			expect(componentState).to.deep.equal({key: 'value'});
+			expect(stateConstant).to.deep.equal({});
+		});
+	}),
+
+
 	describe('Lifecycle DOM Timing', () => {
 		it('should be invoked when dom does (DidMount, WillUnmount) or does not (WillMount, DidUnmount) exist', () => {
 			let setState;