provide optional targetWindow prop

README.md

@@ -129,6 +129,11 @@ Keys of media query objects are camel-cased and numeric values automatically get
 See the [json2mq docs](https://github.com/akiran/json2mq/blob/master/README.md#usage) for more
 examples of queries you can construct using objects.
 
+An optional `targetWindow` prop can be specified if you want the `query` to be
+evaluated against a different window object than the one the code is running in.
+This can be useful for example if you are rendering part of your component tree
+to an iframe or [a popup window](https://hackernoon.com/using-a-react-16-portal-to-do-something-cool-2a2d627b0202).
+
 If you're curious about how react-media differs from
 [react-responsive](https://github.com/contra/react-responsive), please see
 [this comment](https://github.com/ReactTraining/react-media/issues/70#issuecomment-347774260).

modules/Media.js

@@ -14,7 +14,8 @@ class Media extends React.Component {
       PropTypes.arrayOf(PropTypes.object.isRequired)
     ]).isRequired,
     render: PropTypes.func,
-    children: PropTypes.oneOfType([PropTypes.node, PropTypes.func])
+    children: PropTypes.oneOfType([PropTypes.node, PropTypes.func]),
+    targetWindow: PropTypes.object
   };
 
   static defaultProps = {
@@ -28,13 +29,20 @@ class Media extends React.Component {
   updateMatches = () => this.setState({ matches: this.mediaQueryList.matches });
 
   componentWillMount() {
-    if (typeof window !== "object") return;
-
     let { query } = this.props;
+    const targetWindow = this.props.targetWindow || window;
+
+    if (typeof targetWindow !== "object") return;
+
+    if (!targetWindow.matchMedia) {
+      throw new Error(
+        'You passed a `targetWindow` prop to `Media` that does not have a `matchMedia` function.'
+      );
+    }
 
     if (typeof query !== "string") query = json2mq(query);
 
-    this.mediaQueryList = window.matchMedia(query);
+    this.mediaQueryList = targetWindow.matchMedia(query);
     this.mediaQueryList.addListener(this.updateMatches);
     this.updateMatches();
   }

modules/__tests__/Media-test.js

@@ -122,6 +122,44 @@ describe("A <Media>", () => {
     });
   });
 
+  describe("when a custom targetWindow prop is passed", () => {
+    beforeEach(() => {
+      window.matchMedia = createMockMediaMatcher(true);
+    });
+
+    it("renders its child", () => {
+      const testWindow = {
+        matchMedia: createMockMediaMatcher(false)
+      };
+
+      const element = (
+        <Media query="" targetWindow={testWindow}>
+          {matches => (matches ? <div>hello</div> : <div>goodbye</div>)}
+        </Media>
+      );
+
+      ReactDOM.render(element, node, () => {
+        expect(node.firstChild.innerHTML).toMatch(/goodbye/);
+      });
+    });
+
+    describe("when a non-window prop is passed for targetWindow", () => {
+      it("errors with a useful message", () => {
+        const notAWindow = {};
+
+        const element = (
+          <Media query="" targetWindow={notAWindow}>
+            {matches => (matches ? <div>hello</div> : <div>goodbye</div>)}
+          </Media>
+        );
+
+        expect(() => {
+          ReactDOM.render(element, node, () => {});
+        }).toThrow("does not have a `matchMedia` function");
+      });
+    })
+  });
+
   describe("rendered on the server", () => {
     beforeEach(() => {
       window.matchMedia = createMockMediaMatcher(true);