Add tests for warnings in <StrictMode>

Fixes remix-run#6385

Author: mjackson

packages/react-router-dom/modules/__tests__/StrictMode-test.js

@@ -0,0 +1,49 @@
+import React from "react";
+import ReactDOM from "react-dom";
+
+import { BrowserRouter, HashRouter } from "react-router-dom";
+
+if (React.StrictMode) {
+  describe("in <StrictMode>", () => {
+    const node = document.createElement("div");
+
+    afterEach(() => {
+      ReactDOM.unmountComponentAtNode(node);
+    });
+
+    describe("a <BrowserRouter>", () => {
+      it("does not trigger any warnings", () => {
+        spyOn(console, "error");
+
+        ReactDOM.render(
+          <React.StrictMode>
+            <BrowserRouter />
+          </React.StrictMode>,
+          node
+        );
+
+        expect(console.error).not.toHaveBeenCalled();
+      });
+    });
+
+    describe("a <HashRouter>", () => {
+      it("does not trigger any warnings", () => {
+        spyOn(console, "error");
+
+        ReactDOM.render(
+          <React.StrictMode>
+            <HashRouter />
+          </React.StrictMode>,
+          node
+        );
+
+        expect(console.error).not.toHaveBeenCalled();
+      });
+    });
+  });
+} else {
+  console.log(
+    "Skipping <StrictMode> tests because React %s does not support it",
+    React.version
+  );
+}

packages/react-router/modules/__tests__/StrictMode-test.js

@@ -0,0 +1,65 @@
+import React from "react";
+import ReactDOM from "react-dom";
+import createHistory from "history/createMemoryHistory";
+
+import { MemoryRouter, StaticRouter, Router } from "react-router";
+
+if (React.StrictMode) {
+  describe("in <StrictMode>", () => {
+    const node = document.createElement("div");
+
+    afterEach(() => {
+      ReactDOM.unmountComponentAtNode(node);
+    });
+
+    describe("a <MemoryRouter>", () => {
+      it("does not trigger any warnings", () => {
+        spyOn(console, "error");
+
+        ReactDOM.render(
+          <React.StrictMode>
+            <MemoryRouter />
+          </React.StrictMode>,
+          node
+        );
+
+        expect(console.error).not.toHaveBeenCalled();
+      });
+    });
+
+    describe("a <StaticRouter>", () => {
+      it("does not trigger any warnings", () => {
+        spyOn(console, "error");
+
+        ReactDOM.render(
+          <React.StrictMode>
+            <StaticRouter />
+          </React.StrictMode>,
+          node
+        );
+
+        expect(console.error).not.toHaveBeenCalled();
+      });
+    });
+
+    describe("a <Router>", () => {
+      it("does not trigger any warnings", () => {
+        spyOn(console, "error");
+
+        ReactDOM.render(
+          <React.StrictMode>
+            <Router history={createHistory()} />
+          </React.StrictMode>,
+          node
+        );
+
+        expect(console.error).not.toHaveBeenCalled();
+      });
+    });
+  });
+} else {
+  console.log(
+    "Skipping <StrictMode> tests because React %s does not support it",
+    React.version
+  );
+}