Implement basic version of HTMLCollection

Summary:
This implements a basic version of HTMLCollection that's close to the spec but diverges in some things (e.g.: methods could be called with an instance created through `Object.create`, etc.).

This will be used soon to implement `ReadOnlyElement.children` (behind a flag).

See: https://github.com/react-native-community/discussions-and-proposals/pull/607

Changelog: [internal]

Reviewed By: yungsters

Differential Revision: D44055912

fbshipit-source-id: 37bcd7c12916b95a258e6b2e5717a642f478abdf

packages/react-native/Libraries/DOM/OldStyleCollections/ArrayLikeUtils.js

@@ -0,0 +1,30 @@
+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ * @flow strict
+ */
+
+/**
+ * This definition is different from the current built-in type `$ArrayLike`
+ * provided by Flow, in that this is an interface and that one is an object.
+ *
+ * The difference is important because, when using objects, Flow thinks
+ * a `length` property would be copied over when using the spread operator,
+ * which is incorrect.
+ */
+export interface ArrayLike<T> extends Iterable<T> {
+  // This property should've been read-only as well, but Flow doesn't handle
+  // read-only indexers correctly (thinks reads are writes and fails).
+  [indexer: number]: T;
+  +length: number;
+}
+
+export function* createValueIterator<T>(arrayLike: ArrayLike<T>): Iterator<T> {
+  for (let i = 0; i < arrayLike.length; i++) {
+    yield arrayLike[i];
+  }
+}

packages/react-native/Libraries/DOM/OldStyleCollections/HTMLCollection.js

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+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ * @flow strict
+ */
+
+// flowlint unsafe-getters-setters:off
+
+import type {ArrayLike} from './ArrayLikeUtils';
+
+import {createValueIterator} from './ArrayLikeUtils';
+
+// IMPORTANT: The type definition for this module is defined in `HTMLCollection.js.flow`
+// because Flow only supports indexers in classes in declaration files.
+
+// $FlowIssue[prop-missing] Flow doesn't understand [Symbol.iterator]() {} and thinks this class doesn't implement the Iterable<T> interface.
+export default class HTMLCollection<T> implements Iterable<T>, ArrayLike<T> {
+  _length: number;
+
+  /**
+   * Use `createHTMLCollection` to create instances of this class.
+   *
+   * @private This is not defined in the declaration file, so users will not see
+   *          the signature of the constructor.
+   */
+  constructor(elements: $ReadOnlyArray<T>) {
+    for (let i = 0; i < elements.length; i++) {
+      Object.defineProperty(this, i, {
+        value: elements[i],
+        enumerable: true,
+        configurable: false,
+        writable: false,
+      });
+    }
+
+    this._length = elements.length;
+  }
+
+  get length(): number {
+    return this._length;
+  }
+
+  item(index: number): T | null {
+    if (index < 0 || index >= this._length) {
+      return null;
+    }
+
+    // assigning to the interface allows us to access the indexer property in a
+    // type-safe way.
+    // eslint-disable-next-line consistent-this
+    const arrayLike: ArrayLike<T> = this;
+    return arrayLike[index];
+  }
+
+  /**
+   * @deprecated Unused in React Native.
+   */
+  namedItem(name: string): T | null {
+    return null;
+  }
+
+  // $FlowIssue[unsupported-syntax] Flow does not support computed properties in classes.
+  [Symbol.iterator](): Iterator<T> {
+    return createValueIterator(this);
+  }
+}
+
+/**
+ * This is an internal method to create instances of `HTMLCollection`,
+ * which avoids leaking its constructor to end users.
+ * We can do that because the external definition of `HTMLCollection` lives in
+ * `HTMLCollection.js.flow`, not here.
+ */
+export function createHTMLCollection<T>(
+  elements: $ReadOnlyArray<T>,
+): HTMLCollection<T> {
+  return new HTMLCollection(elements);
+}

packages/react-native/Libraries/DOM/OldStyleCollections/HTMLCollection.js.flow

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+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ * @flow strict
+ */
+
+import type {ArrayLike} from './ArrayLikeUtils';
+
+declare export default class HTMLCollection<+T>
+  implements Iterable<T>, ArrayLike<T>
+{
+  // This property should've been read-only as well, but Flow doesn't handle
+  // read-only indexers correctly (thinks reads are writes and fails).
+  [index: number]: T;
+  +length: number;
+  item(index: number): T | null;
+  namedItem(name: string): T | null;
+  @@iterator(): Iterator<T>;
+}
+
+declare export function createHTMLCollection<T>(
+  elements: $ReadOnlyArray<T>,
+): HTMLCollection<T>;

packages/react-native/Libraries/DOM/OldStyleCollections/__tests__/HTMLCollection-test.js

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+/**
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @flow strict-local
+ * @format
+ * @oncall react_native
+ */
+
+import {createHTMLCollection} from '../HTMLCollection';
+
+describe('HTMLCollection', () => {
+  it('provides an array-like interface', () => {
+    const collection = createHTMLCollection(['a', 'b', 'c']);
+
+    expect(collection[0]).toBe('a');
+    expect(collection[1]).toBe('b');
+    expect(collection[2]).toBe('c');
+    expect(collection[3]).toBe(undefined);
+    expect(collection.length).toBe(3);
+  });
+
+  it('is immutable (loose mode)', () => {
+    const collection = createHTMLCollection(['a', 'b', 'c']);
+
+    collection[0] = 'replacement';
+    expect(collection[0]).toBe('a');
+
+    // $FlowExpectedError[cannot-write]
+    collection.length = 100;
+    expect(collection.length).toBe(3);
+  });
+
+  it('is immutable (strict mode)', () => {
+    'use strict';
+
+    const collection = createHTMLCollection(['a', 'b', 'c']);
+
+    expect(() => {
+      collection[0] = 'replacement';
+    }).toThrow(TypeError);
+    expect(collection[0]).toBe('a');
+
+    expect(() => {
+      // $FlowExpectedError[cannot-write]
+      collection.length = 100;
+    }).toThrow(TypeError);
+    expect(collection.length).toBe(3);
+  });
+
+  it('can be converted to an array through common methods', () => {
+    const collection = createHTMLCollection(['a', 'b', 'c']);
+
+    expect(Array.from(collection)).toEqual(['a', 'b', 'c']);
+    expect([...collection]).toEqual(['a', 'b', 'c']);
+  });
+
+  it('can be traversed with for-of', () => {
+    const collection = createHTMLCollection(['a', 'b', 'c']);
+
+    let i = 0;
+    for (const value of collection) {
+      expect(value).toBe(collection[i]);
+      i++;
+    }
+  });
+
+  describe('item()', () => {
+    it('returns elements at the specified position, or null', () => {
+      const collection = createHTMLCollection(['a', 'b', 'c']);
+
+      expect(collection.item(0)).toBe('a');
+      expect(collection.item(1)).toBe('b');
+      expect(collection.item(2)).toBe('c');
+      expect(collection.item(3)).toBe(null);
+    });
+  });
+});