Tighten type config from TS “5.x” > “6.x” change.

CHANGELOG.md

@@ -7,6 +7,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 ## [Unreleased]
 
 ### Changed
+- Re-enable strict type errors for `strictNullChecks`,
+  `useUnknownInCatchVariables`, and `strictFunctionTypes` (#357).
 - Bump devDependencies (eslint 10, typescript 6, eslint-plugin-jsdoc 62, etc.).
 
 ## [2.0.2] - 2026-03-10

tsconfig.json

@@ -21,10 +21,7 @@
     "declarationMap": true,
     "noEmitOnError": true,
     "module": "NodeNext", // Approximates browser target.
-    // TODO: #357: Fix strict type errors.
-    "noImplicitAny": false,
-    "strictNullChecks": false,
-    "useUnknownInCatchVariables": false,
-    "strictFunctionTypes": false
+    // TODO: #357: Fix implicit any types.
+    "noImplicitAny": false
   }
 }

types/x-element.d.ts

@@ -67,13 +67,13 @@ export default class XElement extends HTMLElement {
     static get properties(): {
         [key: string]: {
             type?: (new (...args: unknown[]) => unknown) | undefined;
-            attribute?: string;
-            input?: string[];
-            compute?: (...args: unknown[]) => unknown;
-            observe?: (host: HTMLElement, value: unknown, oldValue: unknown) => any;
-            reflect?: boolean;
-            internal?: boolean;
-            readOnly?: boolean;
+            attribute?: string | undefined;
+            input?: string[] | undefined;
+            compute?: ((...args: unknown[]) => unknown) | undefined;
+            observe?: ((host: HTMLElement, value: unknown, oldValue: unknown) => any) | undefined;
+            reflect?: boolean | undefined;
+            internal?: boolean | undefined;
+            readOnly?: boolean | undefined;
             initial?: unknown | (() => unknown);
             default?: unknown | (() => unknown);
         };
@@ -131,7 +131,7 @@ export default class XElement extends HTMLElement {
     static #validateProperties(constructor: any, properties: any, entries: any): void;
     static #validateProperty(constructor: any, key: any, property: any): void;
     static #validatePropertyAttribute(constructor: any, key: any, property: any, attribute: any): void;
-    static #propertyIsCyclic(property: any, inputMap: any, seen?: Set<any>): boolean;
+    static #propertyIsCyclic(property: any, inputMap: any, seen?: Set<any>): true | undefined;
     static #validateListeners(constructor: any, listeners: any, entries: any): void;
     static #mutateProperty(constructor: any, propertyMap: any, key: any, property: any): void;
     static #addPropertyInitial(constructor: any, property: any): void;

types/x-parser.d.ts

@@ -36,11 +36,11 @@ export class XParser {
     static #boundAttributeMalformed: RegExp;
     static #boundPropertyMalformed: RegExp;
     static #danglingQuoteMalformed: RegExp;
-    static #getErrorMessage(key: any): "Could not parse template markup (at template start)." | "Could not parse template markup (after text content)." | "Could not parse template markup (after a comment)." | "Could not parse template markup (after interpolated content)." | "Could not parse template markup (after a spacing within start tag)." | "Could not parse template markup (after a start tag)." | "Could not parse template markup (after a boolean attribute interpolation in a start tag)." | "Could not parse template markup (after a defined attribute interpolation in a start tag)." | "Could not parse template markup (after an attribute interpolation in a start tag)." | "Could not parse template markup (after a property interpolation in a start tag)." | "Could not parse template markup (after an end tag)." | "Invalid tag name - refer to https://developer.mozilla.org/en-US/docs/Web/API/CustomElementRegistry/define#valid_custom_element_names)." | "Invalid tag whitespace (extraneous whitespace in start tag)." | "Invalid start tag (extraneous whitespace at close of start tag)." | "Invalid end tag." | "Invalid tag attribute (must use kebab-case names and double-quoted values)." | "Invalid tag attribute interpolation (must use kebab-case names and double-quoted values)." | "Invalid tag property interpolation (must use kebab-case names and double-quoted values)." | "Invalid closing quote on tag attribute or property." | "CDATA sections are not supported. Use character references instead: https://developer.mozilla.org/en-US/docs/Glossary/Character_reference." | "Bad escape in tagged template string. Use “&bsol;” for “\\”, “&dollar;” for “\\$”, and “&grave;” for “\\`”. All character references are supported: https://developer.mozilla.org/en-US/docs/Glossary/Character_reference." | "Ambiguous ampersand character or invalid hexadecimal character reference." | "Invalid comment. Comments cannot start with “>” or “->” characters, they cannot include a set of “--” characters, and they cannot end with a “-” character." | "Unsupported native tag - supported native tags are listed here: https://github.com/Netflix/x-element/blob/main/doc/TEMPLATES.md#supported-native-tags." | "Invalid end tag (all non-void start tags much have matching end tags)." | "Unsupported <textarea> interpolation. Interpolation must be exact (<textarea>${…}</textarea>)." | "Unsupported declarative shadow root on <template>. The “shadowrootmode” attribute is not supported." | "Missing closing quote on bound attribute or property.";
-    static #getErrorMessageKeyFromValue(value: any): "#100" | "#101" | "#102" | "#103" | "#104" | "#105" | "#106" | "#107" | "#108" | "#109" | "#110";
-    static #getErrorMessageKeyFromValueMalformed(valueMalformed: any): "#120" | "#121" | "#122" | "#123" | "#124" | "#125" | "#126" | "#127";
-    static #getErrorMessageKeyFromValueForbidden(valueForbidden: any): string;
-    static #getErrorMessageKeyFromErrorName(errorName: any): "#150" | "#151" | "#152" | "#153" | "#154" | "#155" | "#156" | "#157";
+    static #getErrorMessage(key: any): "Could not parse template markup (at template start)." | "Could not parse template markup (after text content)." | "Could not parse template markup (after a comment)." | "Could not parse template markup (after interpolated content)." | "Could not parse template markup (after a spacing within start tag)." | "Could not parse template markup (after a start tag)." | "Could not parse template markup (after a boolean attribute interpolation in a start tag)." | "Could not parse template markup (after a defined attribute interpolation in a start tag)." | "Could not parse template markup (after an attribute interpolation in a start tag)." | "Could not parse template markup (after a property interpolation in a start tag)." | "Could not parse template markup (after an end tag)." | "Invalid tag name - refer to https://developer.mozilla.org/en-US/docs/Web/API/CustomElementRegistry/define#valid_custom_element_names)." | "Invalid tag whitespace (extraneous whitespace in start tag)." | "Invalid start tag (extraneous whitespace at close of start tag)." | "Invalid end tag." | "Invalid tag attribute (must use kebab-case names and double-quoted values)." | "Invalid tag attribute interpolation (must use kebab-case names and double-quoted values)." | "Invalid tag property interpolation (must use kebab-case names and double-quoted values)." | "Invalid closing quote on tag attribute or property." | "CDATA sections are not supported. Use character references instead: https://developer.mozilla.org/en-US/docs/Glossary/Character_reference." | "Bad escape in tagged template string. Use “&bsol;” for “\\”, “&dollar;” for “\\$”, and “&grave;” for “\\`”. All character references are supported: https://developer.mozilla.org/en-US/docs/Glossary/Character_reference." | "Ambiguous ampersand character or invalid hexadecimal character reference." | "Invalid comment. Comments cannot start with “>” or “->” characters, they cannot include a set of “--” characters, and they cannot end with a “-” character." | "Unsupported native tag - supported native tags are listed here: https://github.com/Netflix/x-element/blob/main/doc/TEMPLATES.md#supported-native-tags." | "Invalid end tag (all non-void start tags much have matching end tags)." | "Unsupported <textarea> interpolation. Interpolation must be exact (<textarea>${…}</textarea>)." | "Unsupported declarative shadow root on <template>. The “shadowrootmode” attribute is not supported." | "Missing closing quote on bound attribute or property." | undefined;
+    static #getErrorMessageKeyFromValue(value: any): "#100" | "#101" | "#102" | "#103" | "#104" | "#105" | "#106" | "#107" | "#108" | "#109" | "#110" | undefined;
+    static #getErrorMessageKeyFromValueMalformed(valueMalformed: any): "#120" | "#121" | "#122" | "#123" | "#124" | "#125" | "#126" | "#127" | undefined;
+    static #getErrorMessageKeyFromValueForbidden(valueForbidden: any): "#140" | undefined;
+    static #getErrorMessageKeyFromErrorName(errorName: any): "#150" | "#151" | "#152" | "#153" | "#154" | "#155" | "#156" | "#157" | undefined;
     static #try(string: any, stringIndex: any, ...values: any[]): any;
     static #forbiddenTransition(string: any, stringIndex: any, value: any): any;
     static #invalidTransition(string: any, stringIndex: any, value: any): any;

types/x-template.d.ts

@@ -1,3 +1,3 @@
-export const render: (...args: unknown[]) => unknown;
-export const html: (...args: unknown[]) => unknown;
+export const render: (container: HTMLElement, rawResult: unknown) => void;
+export const html: (strings: string[], ...values: unknown[]) => unknown;
 //# sourceMappingURL=x-template.d.ts.map

x-parser.js

@@ -1069,7 +1069,13 @@ export class XParser {
               break;
             }
           }
-          stringIndex = nextStringIndex; // Update out pointer from our pattern match.
+          // Cast: nextStringIndex is guaranteed non-null here — it’s always
+          //  set from a regex match (value.lastIndex) before reaching this
+          //  point. TypeScript cannot prove this because the assignment happens
+          //  inside a switch-case that doesn’t cover every value of
+          //  nextStringIndex’s type. A zero-cost cast is preferred over a
+          //  runtime guard in this hot loop.
+          stringIndex = /** @type {number} */ (nextStringIndex); // Update out pointer from our pattern match.
           nextStringIndex = null;
         }
         stringsIndex++;
@@ -1078,10 +1084,13 @@ export class XParser {
       XParser.#validateExit(value, tagName);
     } catch (error) {
       // Roughly match the conventions for “onToken”.
-      const index = stringsIndex;
-      const start = stringIndex;
-      const end = nextStringIndex;
-      error[XParser.#errorContextKey] = { index, start, end };
+      if (typeof error === 'object' && error !== null) {
+        // Narrow from "unknown" to attach debug context via Object.assign.
+        const index = stringsIndex;
+        const start = stringIndex;
+        const end = nextStringIndex;
+        Object.assign(error, { [XParser.#errorContextKey]: { index, start, end } });
+      }
       throw error;
     }
   }

x-template.js

@@ -36,7 +36,7 @@ class TemplateEngine {
 
   /**
    * Default template engine interface — what you get inside “template”.
-   * @type {{[key: string]: (...args: unknown[]) => unknown}}
+   * @type {{ render: (container: HTMLElement, rawResult: unknown) => void, html: (strings: string[], ...values: unknown[]) => unknown }}
    */
   static interface = Object.freeze({
     render: TemplateEngine.render,