diff --git a/compat/test/browser/testUtils.js b/compat/test/browser/testUtils.js
index 04ed784717..8746cf1236 100644
--- a/compat/test/browser/testUtils.js
+++ b/compat/test/browser/testUtils.js
@@ -10,13 +10,13 @@ export function getSymbol(name, fallback) {
 		// eslint-disable-next-line
 		if (
 			Function.prototype.toString
-				.call(eval('Symbol.for'))
+				.call((0, eval)('Symbol.for'))
 				.match(/\[native code\]/)
 		) {
 			// Concatenate these string literals to prevent the test
 			// harness and/or Babel from modifying the symbol value.
 			// eslint-disable-next-line
-			out = eval('Sym' + 'bol.for("' + name + '")');
+			out = (0, eval)('Sym' + 'bol.for("' + name + '")');
 		}
 	} catch (e) {}
 
diff --git a/package-lock.json b/package-lock.json
index d886e43b1d..5d3b1bdc58 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -20,6 +20,7 @@
         "@types/chai": "^4.1.2",
         "@types/mocha": "^5.0.0",
         "@types/node": "^14.14.10",
+        "@types/sinon": "^9.0.11",
         "babel-plugin-istanbul": "^6.0.0",
         "babel-plugin-transform-async-to-promises": "^0.8.15",
         "babel-plugin-transform-rename-properties": "0.1.0",
@@ -2360,6 +2361,21 @@
         "@types/node": "*"
       }
     },
+    "node_modules/@types/sinon": {
+      "version": "9.0.11",
+      "resolved": "https://registry.npmjs.org/@types/sinon/-/sinon-9.0.11.tgz",
+      "integrity": "sha512-PwP4UY33SeeVKodNE37ZlOsR9cReypbMJOhZ7BVE0lB+Hix3efCOxiJWiE5Ia+yL9Cn2Ch72EjFTRze8RZsNtg==",
+      "dev": true,
+      "dependencies": {
+        "@types/sinonjs__fake-timers": "*"
+      }
+    },
+    "node_modules/@types/sinonjs__fake-timers": {
+      "version": "8.1.5",
+      "resolved": "https://registry.npmjs.org/@types/sinonjs__fake-timers/-/sinonjs__fake-timers-8.1.5.tgz",
+      "integrity": "sha512-mQkU2jY8jJEF7YHjHvsQO8+3ughTL1mcnn96igfhONmR+fUPSKIkefQYpSe8bsly2Ep7oQbn/6VG5/9/0qcArQ==",
+      "dev": true
+    },
     "node_modules/@types/ua-parser-js": {
       "version": "0.7.36",
       "resolved": "https://registry.npmjs.org/@types/ua-parser-js/-/ua-parser-js-0.7.36.tgz",
@@ -3958,9 +3974,9 @@
       }
     },
     "node_modules/caniuse-lite": {
-      "version": "1.0.30001383",
-      "resolved": "https://registry.npmjs.org/caniuse-lite/-/caniuse-lite-1.0.30001383.tgz",
-      "integrity": "sha512-swMpEoTp5vDoGBZsYZX7L7nXHe6dsHxi9o6/LKf/f0LukVtnrxly5GVb/fWdCDTqi/yw6Km6tiJ0pmBacm0gbg==",
+      "version": "1.0.30001614",
+      "resolved": "https://registry.npmjs.org/caniuse-lite/-/caniuse-lite-1.0.30001614.tgz",
+      "integrity": "sha512-jmZQ1VpmlRwHgdP1/uiKzgiAuGOfLEJsYFP4+GBou/QQ4U6IOJCB4NP1c+1p9RGLpwObcT94jA5/uO+F1vBbog==",
       "dev": true,
       "funding": [
         {
@@ -3970,6 +3986,10 @@
         {
           "type": "tidelift",
           "url": "https://tidelift.com/funding/github/npm/caniuse-lite"
+        },
+        {
+          "type": "github",
+          "url": "https://github.com/sponsors/ai"
         }
       ]
     },
@@ -17789,6 +17809,21 @@
         "@types/node": "*"
       }
     },
+    "@types/sinon": {
+      "version": "9.0.11",
+      "resolved": "https://registry.npmjs.org/@types/sinon/-/sinon-9.0.11.tgz",
+      "integrity": "sha512-PwP4UY33SeeVKodNE37ZlOsR9cReypbMJOhZ7BVE0lB+Hix3efCOxiJWiE5Ia+yL9Cn2Ch72EjFTRze8RZsNtg==",
+      "dev": true,
+      "requires": {
+        "@types/sinonjs__fake-timers": "*"
+      }
+    },
+    "@types/sinonjs__fake-timers": {
+      "version": "8.1.5",
+      "resolved": "https://registry.npmjs.org/@types/sinonjs__fake-timers/-/sinonjs__fake-timers-8.1.5.tgz",
+      "integrity": "sha512-mQkU2jY8jJEF7YHjHvsQO8+3ughTL1mcnn96igfhONmR+fUPSKIkefQYpSe8bsly2Ep7oQbn/6VG5/9/0qcArQ==",
+      "dev": true
+    },
     "@types/ua-parser-js": {
       "version": "0.7.36",
       "resolved": "https://registry.npmjs.org/@types/ua-parser-js/-/ua-parser-js-0.7.36.tgz",
@@ -19003,9 +19038,9 @@
       }
     },
     "caniuse-lite": {
-      "version": "1.0.30001383",
-      "resolved": "https://registry.npmjs.org/caniuse-lite/-/caniuse-lite-1.0.30001383.tgz",
-      "integrity": "sha512-swMpEoTp5vDoGBZsYZX7L7nXHe6dsHxi9o6/LKf/f0LukVtnrxly5GVb/fWdCDTqi/yw6Km6tiJ0pmBacm0gbg==",
+      "version": "1.0.30001614",
+      "resolved": "https://registry.npmjs.org/caniuse-lite/-/caniuse-lite-1.0.30001614.tgz",
+      "integrity": "sha512-jmZQ1VpmlRwHgdP1/uiKzgiAuGOfLEJsYFP4+GBou/QQ4U6IOJCB4NP1c+1p9RGLpwObcT94jA5/uO+F1vBbog==",
       "dev": true
     },
     "capital-case": {
diff --git a/package.json b/package.json
index 8042ccb03f..c5f235ba4a 100644
--- a/package.json
+++ b/package.json
@@ -270,6 +270,7 @@
     "@types/chai": "^4.1.2",
     "@types/mocha": "^5.0.0",
     "@types/node": "^14.14.10",
+    "@types/sinon": "^9.0.11",
     "babel-plugin-istanbul": "^6.0.0",
     "babel-plugin-transform-async-to-promises": "^0.8.15",
     "babel-plugin-transform-rename-properties": "0.1.0",
diff --git a/test/browser/getDomSibling.test.js b/test/browser/getDomSibling.test.js
index ee12f3224a..66a6aa8921 100644
--- a/test/browser/getDomSibling.test.js
+++ b/test/browser/getDomSibling.test.js
@@ -5,7 +5,7 @@ import { setupScratch, teardown } from '../_util/helpers';
 /** @jsx createElement */
 
 describe('getDomSibling', () => {
-	/** @type {import('../../src/internal').PreactElement} */
+	/** @type {HTMLDivElement} */
 	let scratch;
 
 	const getRoot = dom => dom._children;
diff --git a/test/extensions.d.ts b/test/extensions.d.ts
index c810c3bc8f..e86d5a6177 100644
--- a/test/extensions.d.ts
+++ b/test/extensions.d.ts
@@ -1,5 +1,16 @@
-declare module Chai {
-	export interface Assertion {
-		equalNode(node: Node | null, message?: string): void;
+declare global {
+	namespace Chai {
+		interface Assertion {
+			equalNode(node: Node | null, message?: string): void;
+			called: Assertion;
+			calledOnce: Assertion;
+			calledTwice: Assertion;
+			calledWith(...args: any[]): Assertion;
+			calledWithMatch(...args: Record<string, unknown>[]): Assertion;
+		}
 	}
+	var expect: Chai.ExpectStatic;
+	var sinon: Sinon;
 }
+
+export {};