diff --git a/definitelyTypedTests/package-lock.json b/definitelyTypedTests/package-lock.json
index 91fd186e8..623c5ea5c 100644
--- a/definitelyTypedTests/package-lock.json
+++ b/definitelyTypedTests/package-lock.json
@@ -865,11 +865,24 @@
       "resolved": "https://registry.npmjs.org/exit/-/exit-0.1.2.tgz",
       "integrity": "sha1-BjJjj42HfMghB9MKD/8aF8uhzQw="
     },
+    "function-bind": {
+      "version": "1.1.1",
+      "resolved": "https://registry.npmjs.org/function-bind/-/function-bind-1.1.1.tgz",
+      "integrity": "sha512-yIovAzMX49sF8Yl58fSCWJ5svSLuaibPxXQJFLmBObTuCr0Mf1KiPopGM9NiFjiYBCbfaa2Fh6breQ6ANVTI0A=="
+    },
     "globals": {
       "version": "9.18.0",
       "resolved": "https://registry.npmjs.org/globals/-/globals-9.18.0.tgz",
       "integrity": "sha512-S0nG3CLEQiY/ILxqtztTWH/3iRRdyBLw6KMDxnKMchrtbj2OFmehVh0WUCfW3DUrIgx/qFrJPICrq4Z4sTR9UQ=="
     },
+    "has": {
+      "version": "1.0.3",
+      "resolved": "https://registry.npmjs.org/has/-/has-1.0.3.tgz",
+      "integrity": "sha512-f2dvO0VU6Oej7RkWJGrehjbzMAjFp5/VKPp5tTpWIV4JHHZK1/BxbFRtf/siA2SWTe09caDmVtYYzWEIbBS4zw==",
+      "requires": {
+        "function-bind": "^1.1.1"
+      }
+    },
     "has-ansi": {
       "version": "2.0.0",
       "resolved": "https://registry.npmjs.org/has-ansi/-/has-ansi-2.0.0.tgz",
@@ -895,6 +908,14 @@
         "loose-envify": "^1.0.0"
       }
     },
+    "is-core-module": {
+      "version": "2.2.0",
+      "resolved": "https://registry.npmjs.org/is-core-module/-/is-core-module-2.2.0.tgz",
+      "integrity": "sha512-XRAfAdyyY5F5cOXn7hYQDqh2Xmii+DEfIcQGxK/uNwMHhIkPWO0g8msXcbzLe+MpGoR951MlqM/2iIlU4vKDdQ==",
+      "requires": {
+        "has": "^1.0.3"
+      }
+    },
     "is-finite": {
       "version": "1.1.0",
       "resolved": "https://registry.npmjs.org/is-finite/-/is-finite-1.1.0.tgz",
@@ -1050,10 +1071,11 @@
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     },
     "resolve": {
-      "version": "1.15.1",
-      "resolved": "https://registry.npmjs.org/resolve/-/resolve-1.15.1.tgz",
-      "integrity": "sha512-84oo6ZTtoTUpjgNEr5SJyzQhzL72gaRodsSfyxC/AXRvwu0Yse9H8eF9IpGo7b8YetZhlI6v7ZQ6bKBFV/6S7w==",
+      "version": "1.19.0",
+      "resolved": "https://registry.npmjs.org/resolve/-/resolve-1.19.0.tgz",
+      "integrity": "sha512-rArEXAgsBG4UgRGcynxWIWKFvh/XZCcS8UJdHhwy91zwAvCZIbcs+vAbflgBnNjYMs/i/i+/Ux6IZhML1yPvxg==",
       "requires": {
+        "is-core-module": "^2.1.0",
         "path-parse": "^1.0.6"
       }
     },
@@ -1117,22 +1139,22 @@
       "integrity": "sha1-yy4SAwZ+DI3h9hQJS5/kVwTqYAM="
     },
     "ttypescript": {
-      "version": "1.5.10",
-      "resolved": "https://registry.npmjs.org/ttypescript/-/ttypescript-1.5.10.tgz",
-      "integrity": "sha512-Hk7TRej1hM+p+Fo+Pyb/XK9pe9CAt3Sh5n5YRutxFS8hUgkh2u1Vd2K40kMcNP3WYhiVFBMqXwM/2E8O95Ep6g==",
+      "version": "1.5.12",
+      "resolved": "https://registry.npmjs.org/ttypescript/-/ttypescript-1.5.12.tgz",
+      "integrity": "sha512-1ojRyJvpnmgN9kIHmUnQPlEV1gq+VVsxVYjk/NfvMlHSmYxjK5hEvOOU2MQASrbekTUiUM7pR/nXeCc8bzvMOQ==",
       "requires": {
-        "resolve": "^1.9.0"
+        "resolve": ">=1.9.0"
       }
     },
     "typescript": {
-      "version": "3.9.7",
-      "resolved": "https://registry.npmjs.org/typescript/-/typescript-3.9.7.tgz",
-      "integrity": "sha512-BLbiRkiBzAwsjut4x/dsibSTB6yWpwT5qWmC2OfuCg3GgVQCSgMs4vEctYPhsaGtd0AeuuHMkjZ2h2WG8MSzRw=="
+      "version": "4.1.3",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-4.1.3.tgz",
+      "integrity": "sha512-B3ZIOf1IKeH2ixgHhj6la6xdwR9QrLC5d1VKeCSY4tvkqhF2eqd9O7txNlS0PO3GrBAFIdr3L1ndNwteUbZLYg=="
     },
     "upath": {
-      "version": "1.2.0",
-      "resolved": "https://registry.npmjs.org/upath/-/upath-1.2.0.tgz",
-      "integrity": "sha512-aZwGpamFO61g3OlfT7OQCHqhGnW43ieH9WZeP7QxN/G/jS4jfqUkZxoryvJgVPEcrl5NL/ggHsSmLMHuH64Lhg=="
+      "version": "2.0.1",
+      "resolved": "https://registry.npmjs.org/upath/-/upath-2.0.1.tgz",
+      "integrity": "sha512-1uEe95xksV1O0CYKXo8vQvN1JEbtJp7lb7C5U9HMsIp6IVwntkH/oNUzyVNQSd4S1sYk2FpSSW44FqMc8qee5w=="
     },
     "which": {
       "version": "1.3.1",
diff --git a/definitelyTypedTests/package.json b/definitelyTypedTests/package.json
index 55c1bf937..85085cb21 100644
--- a/definitelyTypedTests/package.json
+++ b/definitelyTypedTests/package.json
@@ -18,8 +18,8 @@
   "dependencies": {
     "cross-var": "^1.1.0",
     "dotenv": "^8.2.0",
-    "ttypescript": "^1.5.10",
-    "typescript": "^3.9.7",
-    "upath": "^1.2.0"
+    "ttypescript": "^1.5.12",
+    "typescript": "^4.1.3",
+    "upath": "^2.0.1"
   }
 }
diff --git a/package-lock.json b/package-lock.json
index b429aaf7a..184fbbee1 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -12665,9 +12665,9 @@
       "dev": true
     },
     "typescript": {
-      "version": "3.9.7",
-      "resolved": "https://registry.npmjs.org/typescript/-/typescript-3.9.7.tgz",
-      "integrity": "sha512-BLbiRkiBzAwsjut4x/dsibSTB6yWpwT5qWmC2OfuCg3GgVQCSgMs4vEctYPhsaGtd0AeuuHMkjZ2h2WG8MSzRw==",
+      "version": "4.1.3",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-4.1.3.tgz",
+      "integrity": "sha512-B3ZIOf1IKeH2ixgHhj6la6xdwR9QrLC5d1VKeCSY4tvkqhF2eqd9O7txNlS0PO3GrBAFIdr3L1ndNwteUbZLYg==",
       "dev": true
     },
     "typical": {
diff --git a/package.json b/package.json
index 80dd03ab0..f9f725f77 100644
--- a/package.json
+++ b/package.json
@@ -87,7 +87,7 @@
         "ts-node": "^9.1.1",
         "tsconfig-paths": "^3.9.0",
         "ttypescript": "1.5.12",
-        "typescript": "^3.9.7",
+        "typescript": "^4.1.3",
         "webpack": "^5.10.1",
         "webpack-cli": "^4.2.0",
         "webpack-merge": "^5.7.0",
@@ -99,7 +99,7 @@
         "winston": "^3.3.3"
     },
     "peerDependencies": {
-        "typescript": "^3.9.5"
+        "typescript": "^4.0.2"
     },
     "husky": {
         "hooks": {
diff --git a/src/extension/method/function.ts b/src/extension/method/function.ts
index be48ac42b..e03313af0 100644
--- a/src/extension/method/function.ts
+++ b/src/extension/method/function.ts
@@ -2,6 +2,7 @@ export function isFunction(
   functionToCheck: unknown
 ): functionToCheck is Function {
   return (
-    functionToCheck && {}.toString.call(functionToCheck) === '[object Function]'
+    !!functionToCheck &&
+    {}.toString.call(functionToCheck) === '[object Function]'
   );
 }