fix(mock-doc): improve error message when :scope selector is used

src/mock-doc/selector.ts

@@ -1,16 +1,96 @@
 import { MockElement } from './node';
 import jQuery from './third-party/jquery';
 
-export function matches(selector: string, elm: MockElement) {
-  const r = jQuery.find(selector, undefined, undefined, [elm]);
-  return r.length > 0;
+/**
+ * Check whether an element of interest matches a given selector.
+ *
+ * @param selector the selector of interest
+ * @param elm an element within which to find matching elements
+ * @returns whether the element matches the selector
+ */
+export function matches(selector: string, elm: MockElement): boolean {
+  try {
+    const r = jQuery.find(selector, undefined, undefined, [elm]);
+    return r.length > 0;
+  } catch (e) {
+    updateSelectorError(selector, e);
+    throw e;
+  }
 }
 
+/**
+ * Select the first element that matches a given selector
+ *
+ * @param selector the selector of interest
+ * @param elm the element within which to find a matching element
+ * @returns the first matching element, or null if none is found
+ */
 export function selectOne(selector: string, elm: MockElement) {
-  const r = jQuery.find(selector, elm, undefined, undefined);
-  return r[0] || null;
+  try {
+    const r = jQuery.find(selector, elm, undefined, undefined);
+    return r[0] || null;
+  } catch (e) {
+    updateSelectorError(selector, e);
+    throw e;
+  }
 }
 
-export function selectAll(selector: string, elm: MockElement) {
-  return jQuery.find(selector, elm, undefined, undefined);
+/**
+ * Select all elements that match a given selector
+ *
+ * @param selector the selector of interest
+ * @param elm an element within which to find matching elements
+ * @returns all matching elements
+ */
+export function selectAll(selector: string, elm: MockElement): any {
+  try {
+    return jQuery.find(selector, elm, undefined, undefined);
+  } catch (e) {
+    updateSelectorError(selector, e);
+    throw e;
+  }
+}
+
+/**
+ * A manifest of selectors which are known to be problematic in jQuery. See
+ * here to track implementation and support:
+ * https://github.com/jquery/jquery/issues/5111
+ */
+export const PROBLEMATIC_SELECTORS = [':scope', ':where', ':is'] as const;
+
+/**
+ * Given a selector and an error object thrown by jQuery, annotate the
+ * error's message to add some context as to the probable reason for the error.
+ * In particular, if the selector includes a selector which is known to be
+ * unsupported in jQuery, then we know that was likely the cause of the
+ * error.
+ *
+ * @param selector our selector of interest
+ * @param e an error object that was thrown in the course of using jQuery
+ */
+function updateSelectorError(selector: string, e: unknown) {
+  const selectorsPresent = PROBLEMATIC_SELECTORS.filter((s) => selector.includes(s));
+
+  if (selectorsPresent.length > 0 && (e as Error).message) {
+    (e as Error).message =
+      `At present jQuery does not support the ${humanReadableList(selectorsPresent)} ${selectorsPresent.length === 1 ? 'selector' : 'selectors'}.
+If you need this in your test, consider writing an end-to-end test instead.\n` + (e as Error).message;
+  }
+}
+
+/**
+ * Format a list of strings in a 'human readable' way.
+ *
+ * - If one string (['string']), return 'string'
+ * - If two strings (['a', 'b']), return 'a and b'
+ * - If three or more (['a', 'b', 'c']), return 'a, b and c'
+ *
+ * @param items a list of strings to format
+ * @returns a formatted string
+ */
+function humanReadableList(items: string[]): string {
+  if (items.length <= 1) {
+    return items.join('');
+  }
+  return `${items.slice(0, items.length - 1).join(', ')} and ${items[items.length - 1]}`;
 }

src/mock-doc/test/selector.spec.ts

@@ -1,5 +1,6 @@
 import { MockDocument } from '../document';
 import { MockElement } from '../node';
+import { PROBLEMATIC_SELECTORS } from '../selector';
 
 describe('selector', () => {
   it('closest', () => {
@@ -225,4 +226,51 @@ describe('selector', () => {
     const q2 = div.querySelector('span.c a');
     expect(q2.tagName).toBe('A');
   });
+
+  it.each(PROBLEMATIC_SELECTORS)("should error for '%p' selector", (selector) => {
+    const doc = new MockDocument();
+
+    const expectedMessage = [
+      `At present jQuery does not support the ${selector} selector.`,
+      'If you need this in your test, consider writing an end-to-end test instead.',
+      `Syntax error, unrecognized expression: unsupported pseudo: ${selector.replace(':', '')}`,
+    ].join('\n');
+
+    expect(() => doc.querySelector(selector)).toThrow(expectedMessage);
+    expect(() => doc.querySelectorAll(selector)).toThrow(expectedMessage);
+    expect(() => doc.matches(selector)).toThrow(expectedMessage);
+  });
+
+  it('should error for combinations of problematic selectors', () => {
+    const doc = new MockDocument();
+    expect(() => {
+      doc.querySelector(':scope :is');
+    }).toThrow(
+      [
+        `At present jQuery does not support the :scope and :is selectors.`,
+        'If you need this in your test, consider writing an end-to-end test instead.',
+        `Syntax error, unrecognized expression: unsupported pseudo: scope`,
+      ].join('\n'),
+    );
+
+    expect(() => {
+      doc.querySelector(':is :where');
+    }).toThrow(
+      [
+        `At present jQuery does not support the :where and :is selectors.`,
+        'If you need this in your test, consider writing an end-to-end test instead.',
+        `Syntax error, unrecognized expression: unsupported pseudo: is`,
+      ].join('\n'),
+    );
+
+    expect(() => {
+      doc.querySelector(':scope :is :where');
+    }).toThrow(
+      [
+        `At present jQuery does not support the :scope, :where and :is selectors.`,
+        'If you need this in your test, consider writing an end-to-end test instead.',
+        `Syntax error, unrecognized expression: unsupported pseudo: scope`,
+      ].join('\n'),
+    );
+  });
 });