diff --git a/package-lock.json b/package-lock.json
index 34611f2..ff39413 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -1,12 +1,12 @@
 {
   "name": "@rocicorp/logger",
-  "version": "4.1.0",
+  "version": "5.1.0",
   "lockfileVersion": 2,
   "requires": true,
   "packages": {
     "": {
       "name": "@rocicorp/logger",
-      "version": "4.1.0",
+      "version": "5.1.0",
       "license": "Apache-2.0",
       "devDependencies": {
         "@types/chai": "^4.3.0",
@@ -19,10 +19,7 @@
         "mocha": "^9.2.2",
         "prettier": "^2.6.0",
         "sinon": "^13.0.1",
-        "typescript": "^4.6.2"
-      },
-      "engines": {
-        "node": "^14.13.1 || >=16.0.0"
+        "typescript": "^5.1.3"
       }
     },
     "node_modules/@eslint/eslintrc": {
@@ -2146,16 +2143,16 @@
       }
     },
     "node_modules/typescript": {
-      "version": "4.6.2",
-      "resolved": "https://registry.npmjs.org/typescript/-/typescript-4.6.2.tgz",
-      "integrity": "sha512-HM/hFigTBHZhLXshn9sN37H085+hQGeJHJ/X7LpBWLID/fbc2acUMfU+lGD98X81sKP+pFa9f0DZmCwB9GnbAg==",
+      "version": "5.1.3",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-5.1.3.tgz",
+      "integrity": "sha512-XH627E9vkeqhlZFQuL+UsyAXEnibT0kWR2FWONlr4sTjvxyJYnyefgrkyECLzM5NenmKzRAy2rR/OlYLA1HkZw==",
       "dev": true,
       "bin": {
         "tsc": "bin/tsc",
         "tsserver": "bin/tsserver"
       },
       "engines": {
-        "node": ">=4.2.0"
+        "node": ">=14.17"
       }
     },
     "node_modules/uri-js": {
@@ -3815,9 +3812,9 @@
       "dev": true
     },
     "typescript": {
-      "version": "4.6.2",
-      "resolved": "https://registry.npmjs.org/typescript/-/typescript-4.6.2.tgz",
-      "integrity": "sha512-HM/hFigTBHZhLXshn9sN37H085+hQGeJHJ/X7LpBWLID/fbc2acUMfU+lGD98X81sKP+pFa9f0DZmCwB9GnbAg==",
+      "version": "5.1.3",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-5.1.3.tgz",
+      "integrity": "sha512-XH627E9vkeqhlZFQuL+UsyAXEnibT0kWR2FWONlr4sTjvxyJYnyefgrkyECLzM5NenmKzRAy2rR/OlYLA1HkZw==",
       "dev": true
     },
     "uri-js": {
diff --git a/package.json b/package.json
index d1f7ecc..ccb1c97 100644
--- a/package.json
+++ b/package.json
@@ -4,9 +4,6 @@
   "version": "5.1.0",
   "repository": "github:rocicorp/logger",
   "license": "Apache-2.0",
-  "engines": {
-    "node": "^14.13.1 || >=16.0.0"
-  },
   "scripts": {
     "test": "mocha --ui=tdd out/*.test.js",
     "pretest": "npm run build",
@@ -27,7 +24,7 @@
     "mocha": "^9.2.2",
     "prettier": "^2.6.0",
     "sinon": "^13.0.1",
-    "typescript": "^4.6.2"
+    "typescript": "^5.1.3"
   },
   "type": "module",
   "types": "out/logger.d.ts",
diff --git a/src/logger.ts b/src/logger.ts
index 417b2cd..76c22bb 100644
--- a/src/logger.ts
+++ b/src/logger.ts
@@ -6,10 +6,10 @@
  * only [[error]] will be present.
  */
 export interface OptionalLogger {
-  error?(...args: unknown[]): void;
-  info?(...args: unknown[]): void;
-  debug?(...args: unknown[]): void;
-  flush?(): Promise<void>;
+  error?: ((...args: unknown[]) => void) | undefined;
+  info?: ((...args: unknown[]) => void) | undefined;
+  debug?: ((...args: unknown[]) => void) | undefined;
+  flush?: (() => Promise<void>) | undefined;
 }
 
 /**
@@ -57,9 +57,9 @@ export class TeeLogSink implements LogSink {
 }
 
 export class OptionalLoggerImpl implements OptionalLogger {
-  readonly debug?: (...args: unknown[]) => void = undefined;
-  readonly info?: (...args: unknown[]) => void = undefined;
-  readonly error?: (...args: unknown[]) => void = undefined;
+  readonly debug?: ((...args: unknown[]) => void) | undefined = undefined;
+  readonly info?: ((...args: unknown[]) => void) | undefined = undefined;
+  readonly error?: ((...args: unknown[]) => void) | undefined = undefined;
   readonly flush: () => Promise<void>;
 
   constructor(logSink: LogSink, level: LogLevel = 'info', context?: Context) {
diff --git a/tsconfig.json b/tsconfig.json
index a289d59..3060ecd 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -9,12 +9,10 @@
     "noImplicitReturns": true,
     "noFallthroughCasesInSwitch": true,
     "esModuleInterop": true,
-
-    "importsNotUsedAsValues": "error",
-
+    "exactOptionalPropertyTypes": true,
     "declaration": true,
     "forceConsistentCasingInFileNames": true,
-    "moduleResolution": "node",
+    "moduleResolution": "node16",
     "outDir": "out",
     "allowJs": true,
     // esnext for Object.fromEntries