Fix type assertion

index.js

@@ -7,6 +7,11 @@
  * @typedef {null|undefined|TagName|TestFunctionAnything|Array.<TagName|TestFunctionAnything>} Test
  */
 
+/**
+ * @template {Element} T
+ * @typedef {null|undefined|T['tagName']|TestFunctionPredicate<T>|Array.<T['tagName']|TestFunctionPredicate<T>>} PredicateTest
+ */
+
 /**
  * Check if an element passes a test
  *
@@ -22,7 +27,7 @@
  *
  * @template {Element} X
  * @callback TestFunctionPredicate
- * @param {X} element
+ * @param {Element} element
  * @param {number|null|undefined} [index]
  * @param {Parent|null|undefined} [parent]
  * @returns {element is X}
@@ -56,8 +61,9 @@ export const isElement =
    * When a `parent` node is known the `index` of node should also be given.
    *
    * @type {(
-   *   (<T extends Element>(node: unknown, test: T['tagName']|TestFunctionPredicate<T>|Array.<T['tagName']|TestFunctionPredicate<T>>, index?: number, parent?: Parent, context?: unknown) => node is T) &
-   *   ((node?: unknown, test?: Test, index?: number, parent?: Parent, context?: unknown) => boolean)
+   *   (() => false) &
+   *   (<T extends Element = Element>(node: unknown, test?: PredicateTest<T>, index?: number, parent?: Parent, context?: unknown) => node is T) &
+   *   ((node: unknown, test: Test, index?: number, parent?: Parent, context?: unknown) => boolean)
    * )}
    */
   (

index.test-d.ts

@@ -30,7 +30,7 @@ const article: Element = {
 const isSection = (element: Element): element is Section =>
   element.tagName === 'section'
 
-isElement()
+expectType<false>(isElement())
 
 /* Missing parameters. */
 expectError(isElement<Section>())
@@ -133,3 +133,35 @@ convertElement()
 convertElement(null)
 convertElement(undefined)
 expectError(convertElement<Article>())
+
+declare const node: unknown
+
+/* Type assertion */
+if (isElement(node)) {
+  expectType<Element>(node)
+}
+
+if (isElement(node, (node): node is Section => node.tagName === 'section')) {
+  expectType<Section>(node)
+}
+
+/**
+ * Should prefer `isElement(node) && node.children.length > 0`,
+ * see below comments for details.
+ */
+if (isElement(node, (node) => node.children.length > 0)) {
+  expectType<unknown>(node)
+} else {
+  expectType<unknown>(node)
+}
+
+/**
+ * This is suggested to be used because `isElement(node)` is a correct assertion,
+ * but with a second param `test` which is not an assertion,
+ * the whole `isElement(node, test)` expression can not be an assertion anymore.
+ */
+if (isElement(node) && node.children.length > 0) {
+  expectType<Element>(node)
+} else {
+  expectType<unknown>(node)
+}