diff --git a/.github/workflows/test.yml b/.github/workflows/test.yml
index d1694ab..2f7bf8c 100644
--- a/.github/workflows/test.yml
+++ b/.github/workflows/test.yml
@@ -22,7 +22,7 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        node: [18]
+        node: [20]
         os: [ubuntu-latest]
 
     steps:
diff --git a/.gitignore b/.gitignore
index fc0322d..d5c1f26 100644
--- a/.gitignore
+++ b/.gitignore
@@ -5,3 +5,7 @@ node_modules
 .DS_Store
 *.vsix
 *.zip
+
+# TypeScript build info files
+.tsbuildinfo
+*/.tsbuildinfo
diff --git a/htmlhint-server/src/server.ts b/htmlhint-server/src/server.ts
index 74bc4ae..152fe28 100644
--- a/htmlhint-server/src/server.ts
+++ b/htmlhint-server/src/server.ts
@@ -19,7 +19,7 @@ interface Settings {
   [key: string]: any;
 }
 
-let settings: Settings = null;
+let settings: Settings | null = null;
 let linter: any = null;
 
 /**
@@ -34,6 +34,20 @@ let htmlhintrcOptions: any = {};
  */
 function getRange(error: htmlhint.Error, lines: string[]): any {
   let line = lines[error.line - 1];
+  if (!line) {
+    // Fallback if line doesn't exist
+    return {
+      start: {
+        line: error.line - 1,
+        character: error.col - 1,
+      },
+      end: {
+        line: error.line - 1,
+        character: error.col - 1,
+      },
+    };
+  }
+
   let isWhitespace = false;
   let curr = error.col;
   while (curr < line.length && !isWhitespace) {
@@ -84,7 +98,7 @@ function makeDiagnostic(
  */
 function getConfiguration(filePath: string): any {
   let options: any;
-  if (settings.htmlhint) {
+  if (settings?.htmlhint) {
     if (
       settings.htmlhint.configFile &&
       settings.htmlhint.options &&
@@ -159,7 +173,7 @@ function findConfigForHtmlFile(base: string) {
 /**
  * Given a path to a .htmlhintrc file, load it into a javascript object and return it.
  */
-function loadConfigurationFile(configFile): any {
+function loadConfigurationFile(configFile: string): any {
   let ruleset: any = null;
   if (fs.existsSync(configFile)) {
     let config = fs.readFileSync(configFile, "utf8");
@@ -170,16 +184,23 @@ function loadConfigurationFile(configFile): any {
   return ruleset;
 }
 
-function getErrorMessage(err: any, document: server.TextDocument): string {
-  let result: string = null;
-  if (typeof err.message === "string" || err.message instanceof String) {
-    result = <string>err.message;
-  } else {
-    result = `An unknown error occurred while validating file: ${server.Files.uriToFilePath(
-      document.uri,
-    )}`;
+function isErrorWithMessage(err: unknown): err is { message: string } {
+  return (
+    typeof err === "object" &&
+    err !== null &&
+    "message" in err &&
+    typeof (err as Record<string, unknown>).message === "string"
+  );
+}
+
+function getErrorMessage(err: unknown, document: server.TextDocument): string {
+  if (isErrorWithMessage(err)) {
+    return err.message;
   }
-  return result;
+
+  return `An unknown error occurred while validating file: ${server.Files.uriToFilePath(
+    document.uri,
+  )}`;
 }
 
 function validateAllTextDocuments(
@@ -284,10 +305,8 @@ function doValidate(
     }
     connection.sendDiagnostics({ uri, diagnostics });
   } catch (err) {
-    let message: string = null;
-    if (typeof err.message === "string" || err.message instanceof String) {
-      message = <string>err.message;
-      throw new Error(message);
+    if (isErrorWithMessage(err)) {
+      throw new Error(err.message);
     }
     throw err;
   }
diff --git a/htmlhint-server/src/tsconfig.json b/htmlhint-server/src/tsconfig.json
index 2facac1..04e2b08 100644
--- a/htmlhint-server/src/tsconfig.json
+++ b/htmlhint-server/src/tsconfig.json
@@ -1,13 +1,37 @@
 {
   "compilerOptions": {
-    "forceConsistentCasingInFileNames": true,
-    "target": "es6",
     "module": "commonjs",
+    "target": "ES2022",
+    "lib": ["ES2022"],
     "moduleResolution": "node",
-    "skipLibCheck": true,
-    "sourceMap": true,
     "outDir": "../../htmlhint/server",
-    "lib": ["es6"]
+    "sourceMap": true,
+    "declaration": true,
+    "declarationMap": true,
+    "strict": true,
+    "noImplicitAny": false,
+    "noImplicitReturns": true,
+    "noImplicitThis": true,
+    "noImplicitOverride": true,
+    "strictNullChecks": false,
+    "strictFunctionTypes": true,
+    "strictBindCallApply": true,
+    "strictPropertyInitialization": false,
+    "alwaysStrict": true,
+    "noUnusedLocals": false,
+    "noUnusedParameters": false,
+    "exactOptionalPropertyTypes": false,
+    "noFallthroughCasesInSwitch": true,
+    "noUncheckedIndexedAccess": false,
+    "noPropertyAccessFromIndexSignature": false,
+    "esModuleInterop": true,
+    "allowSyntheticDefaultImports": true,
+    "resolveJsonModule": true,
+    "forceConsistentCasingInFileNames": true,
+    "skipLibCheck": true,
+    "incremental": true,
+    "tsBuildInfoFile": ".tsbuildinfo"
   },
-  "exclude": ["node_modules"]
+  "include": ["**/*.ts"],
+  "exclude": ["node_modules", "**/*.test.ts", "**/*.spec.ts"]
 }
diff --git a/htmlhint/extension.ts b/htmlhint/extension.ts
index 6af1e21..c40b88e 100644
--- a/htmlhint/extension.ts
+++ b/htmlhint/extension.ts
@@ -26,7 +26,7 @@ export function activate(context: ExtensionContext) {
 
   // Get file types to lint from user settings
   let config = workspace.getConfiguration("htmlhint");
-  let languages: string[] = config.get("documentSelector");
+  let languages: string[] = config.get("documentSelector") || ["html", "htm"];
   let documentSelector = languages.map((language) => ({
     language,
     scheme: "file",
diff --git a/htmlhint/tsconfig.json b/htmlhint/tsconfig.json
index aeee4e3..9f59e41 100644
--- a/htmlhint/tsconfig.json
+++ b/htmlhint/tsconfig.json
@@ -1,12 +1,37 @@
 {
   "compilerOptions": {
     "module": "commonjs",
-    "target": "es6",
+    "target": "ES2022",
+    "lib": ["ES2022", "DOM"],
+    "moduleResolution": "node",
     "outDir": "out",
-    "lib": ["es6", "dom"],
-    "skipLibCheck": true,
     "sourceMap": true,
-    "forceConsistentCasingInFileNames": true
+    "declaration": true,
+    "declarationMap": true,
+    "strict": true,
+    "noImplicitAny": true,
+    "noImplicitReturns": true,
+    "noImplicitThis": true,
+    "noImplicitOverride": true,
+    "strictNullChecks": true,
+    "strictFunctionTypes": true,
+    "strictBindCallApply": true,
+    "strictPropertyInitialization": true,
+    "alwaysStrict": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "exactOptionalPropertyTypes": true,
+    "noFallthroughCasesInSwitch": true,
+    "noUncheckedIndexedAccess": true,
+    "noPropertyAccessFromIndexSignature": false,
+    "esModuleInterop": true,
+    "allowSyntheticDefaultImports": true,
+    "resolveJsonModule": true,
+    "forceConsistentCasingInFileNames": true,
+    "skipLibCheck": true,
+    "incremental": true,
+    "tsBuildInfoFile": ".tsbuildinfo"
   },
-  "exclude": ["node_modules", "server"]
+  "include": ["extension.ts", "**/*.ts"],
+  "exclude": ["node_modules", "server", "out", "**/*.test.ts", "**/*.spec.ts"]
 }
diff --git a/test/tsconfig.json b/test/tsconfig.json
index 70a2c12..7a6e29d 100644
--- a/test/tsconfig.json
+++ b/test/tsconfig.json
@@ -1,13 +1,30 @@
 {
   "compilerOptions": {
     "module": "commonjs",
-    "target": "es6",
+    "target": "ES2022",
+    "lib": ["ES2022", "DOM"],
+    "moduleResolution": "node",
     "outDir": "out",
-    "lib": ["es6", "dom"],
-    "types": ["node", "mocha"],
+    "sourceMap": true,
+    "strict": true,
+    "noImplicitAny": true,
+    "noImplicitReturns": true,
+    "noImplicitThis": true,
+    "strictNullChecks": true,
+    "strictFunctionTypes": true,
+    "strictBindCallApply": true,
+    "alwaysStrict": true,
+    "noUnusedLocals": false,
+    "noUnusedParameters": false,
+    "noFallthroughCasesInSwitch": true,
+    "noUncheckedIndexedAccess": false,
     "esModuleInterop": true,
-    "skipLibCheck": true,
-    "sourceMap": true
+    "allowSyntheticDefaultImports": true,
+    "resolveJsonModule": true,
+    "forceConsistentCasingInFileNames": true,
+    "types": ["node", "mocha"],
+    "skipLibCheck": true
   },
-  "include": ["suite/**/*.ts", "runTest.ts"]
+  "include": ["suite/**/*.ts", "runTest.ts"],
+  "exclude": ["node_modules"]
 }