Preact: support ssrSnapshot argument.

package.json

@@ -177,6 +177,7 @@
     "jest": "^27.3.1",
     "jest-esm-jsx-transform": "^1.0.0",
     "preact": "^10.0.0",
+    "preact-render-to-string": "^6.0.3",
     "prettier": "^2.4.1",
     "react": "^18.2.0",
     "react-dom": "^18.2.0",

preact/react-deps.js

@@ -13,11 +13,20 @@ export {
   useContext,
 } from "preact/hooks";
 
+// Copied from:
+// https://github.com/facebook/react/blob/main/packages/shared/ExecutionEnvironment.js
+const canUseDOM = !!(
+  typeof window !== "undefined" &&
+  typeof window.document !== "undefined" &&
+  typeof window.document.createElement !== "undefined"
+);
+
 // TODO: switch to `export { useSyncExternalStore } from "preact/compat"` once we update Preact to >= 10.11.3
 function is(x, y) {
   return (x === y && (x !== 0 || 1 / x === 1 / y)) || (x !== x && y !== y);
 }
-export function useSyncExternalStore(subscribe, getSnapshot) {
+export function useSyncExternalStore(subscribe, getSnapshot, getSSRSnapshot) {
+  if (getSSRSnapshot && !canUseDOM) getSnapshot = getSSRSnapshot;
   const value = getSnapshot();
 
   const [{ _instance }, forceUpdate] = useState({

test/preact-ssr.test.js

@@ -0,0 +1,25 @@
+/**
+ * @jsx h
+ * @jest-environment node
+ */
+
+import { h } from "preact";
+import renderToString from "preact-render-to-string";
+
+// make the library use Preact exports
+jest.mock("../react-deps.js", () => require("../preact/react-deps.js"));
+const { Route, Link, Switch, Router, useLocation } = require("../index.js");
+
+describe("Preact SSR", () => {
+  it("supports SSR", () => {
+    const LocationPrinter = () => `location = ${useLocation()[0]}`;
+
+    const rendered = renderToString(
+      <Router ssrPath="/ssr/preact">
+        <LocationPrinter />
+      </Router>
+    );
+
+    expect(rendered).toBe("location = /ssr/preact");
+  });
+});