Add Array deprecation guides

content/ember/v6/deprecate-ember-array-foundations.md

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+---
+title: Deprecation of @ember/array Foundation APIs
+until: "6.0"
+since: "5.8"
+displayId: array.foundations
+---
+
+The foundational APIs of `@ember/array`, including the `A()` function and the core mixins (`EmberArray`, `MutableArray`, `NativeArray`), are deprecated. These were used to create and extend arrays with Ember's observability. The modern approach is to use native JavaScript arrays, and `TrackedArray` from `tracked-built-ins` when reactivity is needed.
+
+### `A()` Function and Core Mixins
+
+The `A()` function would wrap a native array, making it an `EmberArray`. The `EmberArray` and `MutableArray` mixins could be used to build custom array-like classes.
+
+**Before**
+```javascript
+import { A } from '@ember/array';
+import EmberObject from '@ember/object';
+import { MutableArray } from '@ember/array';
+
+let emberArr = A([1, 2, 3]);
+emberArr.pushObject(4);
+
+const MyArray = EmberObject.extend(MutableArray, {
+  // ... implementation ...
+});
+```
+
+**After**
+Use native arrays for standard array operations. For arrays that need to be tracked for reactivity in components and other classes, use `TrackedArray` from the `tracked-built-ins` addon.
+
+```javascript
+// For a standard array
+let nativeArr = [1, 2, 3];
+nativeArr.push(4);
+
+// For a tracked array
+import { TrackedArray } from 'tracked-built-ins';
+let trackedArr = new TrackedArray([1, 2, 3]);
+trackedArr.push(4); // This mutation is tracked
+```
+
+### Utility Functions: `isArray` and `makeArray`
+
+These functions helped create and check for arrays.
+
+**Before**
+```javascript
+import { isArray, makeArray } from '@ember/array';
+let isArr = isArray([]);
+let ensuredArr = makeArray('hello');
+```
+
+**After**
+Use native JavaScript equivalents.
+
+```javascript
+// isArray() -> Array.isArray()
+let isArr = Array.isArray([]);
+
+// makeArray() -> custom helper or ensure data is correct
+function ensureArray(value) {
+  if (value === null || value === undefined) return [];
+  return Array.isArray(value) ? value : [value];
+}
+let ensuredArr = ensureArray('hello');
+```
+
+### Creating Custom Arrays with Proxies
+
+For advanced use cases where you might have created a custom class based on `MutableArray` to add special behaviors to your array, the modern JavaScript equivalent is to use a `Proxy`. A `Proxy` object allows you to intercept and redefine fundamental operations for a target object (like an array), enabling you to create powerful custom wrappers.
+
+**Example: A Logging Array**
+
+Imagine you want to log every time an item is pushed to an array.
+
+**Before**, you might have done this with `MutableArray`:
+
+```javascript
+import EmberObject from '@ember/object';
+import { MutableArray } from '@ember/array';
+
+const LoggingArray = EmberObject.extend(MutableArray, {
+  // Internal content array
+  _content: null,
+
+  init() {
+    this._super(...arguments);
+    this._content = this._content || [];
+  },
+
+  // Required primitives
+  objectAt(idx) { return this._content[idx]; },
+  get length() { return this._content.length; },
+
+  // Override replace to add logging
+  replace(idx, amt, objects) {
+    if (amt === 0) {
+      console.log(`Adding items: ${objects.join(', ')}`);
+    }
+    this._content.splice(idx, amt, ...objects);
+    this.arrayContentDidChange(idx, amt, objects.length);
+  }
+});
+
+let arr = LoggingArray.create({ _content: [1, 2] });
+arr.pushObject(3); // Logs: "Adding items: 3"
+```
+
+**After**, you can achieve the same result more cleanly by wrapping a `TrackedArray` in a `Proxy`. This allows you to add custom behavior while preserving the reactivity provided by `TrackedArray`.
+
+```javascript
+import { TrackedArray } from 'tracked-built-ins';
+
+function createTrackedLoggingArray(initialItems) {
+  // Start with a TrackedArray instance
+  const trackedArr = new TrackedArray(initialItems);
+
+  return new Proxy(trackedArr, {
+    get(target, prop, receiver) {
+      // Intercept the 'push' method
+      if (prop === 'push') {
+        return function(...args) {
+          console.log(`Adding items via push: ${args.join(', ')}`);
+          // Call the original push method on the TrackedArray
+          // This will trigger reactivity automatically.
+          return target.push(...args);
+        }
+      }
+
+      // Forward all other property access and method calls to the TrackedArray
+      const value = Reflect.get(target, prop, receiver);
+      return typeof value === 'function' ? value.bind(target) : value;
+    },
+
+    set(target, prop, value, receiver) {
+      // Intercept direct index assignment
+      if (!isNaN(parseInt(prop, 10))) {
+        console.log(`Setting index ${prop} to ${value}`);
+      }
+      // Forward the set operation to the TrackedArray to trigger reactivity
+      return Reflect.set(target, prop, value, receiver);
+    }
+  });
+}
+
+// In a component:
+class MyComponent {
+  loggingArray = createTrackedLoggingArray([1, 2]);
+
+  addItem() {
+    this.loggingArray.push(3); // Logs and triggers an update
+  }
+
+  updateItem() {
+    this.loggingArray[0] = 'new value'; // Logs and triggers an update
+  }
+}
+```
+
+This `Proxy` approach is very powerful. By wrapping a `TrackedArray`, you can layer in custom logic while letting it handle the complexities of reactivity. This is the recommended pattern for creating advanced, observable array-like objects in modern Ember.

content/ember/v6/deprecate-ember-array-read-methods.md

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+---
+title: Deprecation of @ember/array Read Methods
+until: "6.0"
+since: "5.8"
+displayId: array.read-methods
+---
+
+All read-only methods and computed properties from `@ember/array` are deprecated. You should use native JavaScript array methods and properties instead. This guide covers the common read-only APIs and their native equivalents.
+
+### `firstObject` and `lastObject`
+
+These computed properties provided safe access to the first and last elements of an array.
+
+**Before**
+```javascript
+import { A } from '@ember/array';
+let arr = A(['a', 'b', 'c']);
+let first = arr.get('firstObject'); // 'a'
+let last = arr.get('lastObject');   // 'c'
+```
+
+**After**
+Use native array bracket notation, or the `at()` method for accessing elements from the end of the array.
+
+```javascript
+let arr = ['a', 'b', 'c'];
+let first = arr[0];
+let last = arr.at(-1);
+```
+
+### `objectAt` and `objectsAt`
+
+These methods provided safe, index-based access to array elements.
+
+**Before**
+```javascript
+import { A } from '@ember/array';
+let arr = A(['a', 'b', 'c']);
+let middle = arr.objectAt(1);     // 'b'
+let some = arr.objectsAt([0, 2]); // ['a', 'c']
+```
+
+**After**
+Use native array bracket notation for `objectAt`. For `objectsAt`, you can use `map`.
+
+```javascript
+let arr = ['a', 'b', 'c'];
+let middle = arr[1];
+let some = [0, 2].map(index => arr[index]);
+```
+
+### `mapBy`, `filterBy`, `rejectBy`, `findBy`
+
+These methods were shortcuts for common mapping and filtering operations on arrays of objects.
+
+**Before**
+```javascript
+import { A } from '@ember/array';
+let users = A([
+  { name: 'John', isActive: true },
+  { name: 'Jane', isActive: false },
+]);
+let names = users.mapBy('name');
+let active = users.filterBy('isActive', true);
+let john = users.findBy('name', 'John');
+```
+
+**After**
+Use the native `map`, `filter`, and `find` methods with arrow functions.
+
+```javascript
+let users = [
+  { name: 'John', isActive: true },
+  { name: 'Jane', isActive: false },
+];
+let names = users.map(user => user.name);
+let active = users.filter(user => user.isActive === true);
+let john = users.find(user => user.name === 'John');
+```
+
+### `uniqBy`
+
+`uniqBy` created a new array with unique elements based on a property.
+
+**Before**
+```javascript
+import { uniqBy } from '@ember/array';
+let users = [{ id: 1 }, { id: 2 }, { id: 1 }];
+let unique = uniqBy(users, 'id');
+```
+
+**After**
+Use a `Map` to efficiently create a unique list.
+
+```javascript
+let users = [{ id: 1 }, { id: 2 }, { id: 1 }];
+let unique = Array.from(
+  users.reduce((map, user) => map.set(user.id, user), new Map()).values()
+);
+```
+

content/ember/v6/deprecate-ember-array-write-methods.md

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+---
+title: Deprecation of @ember/array Write Methods
+until: "6.0"
+since: "5.8"
+displayId: array.write-methods
+---
+
+All methods from `@ember/array` that mutate or "write" to an array are deprecated. This includes observable methods like `pushObject`.
+
+The modern approach is to use `TrackedArray` from the [`tracked-built-ins`](https://github.com/tracked-tools/tracked-built-ins) addon. This class provides a tracked version of the native JavaScript `Array` that can be mutated directly, and these mutations will be tracked automatically.
+
+First, install the addon:
+
+```bash
+ember install tracked-built-ins
+```
+
+### Observable Write Methods
+
+Methods like `pushObject`, `popObject`, `removeObject`, `insertAt`, and `removeAt` were used to modify arrays in a way that Ember's classic observability system could track.
+
+**Before**
+```javascript
+import { A } from '@ember/array';
+let arr = A([1, 2, 3]);
+
+arr.pushObject(4);
+arr.removeAt(1, 1); // remove 1 item at index 1
+```
+
+**After**
+Use `TrackedArray` and mutate it directly with standard JavaScript array methods. Using `TrackedArray` provides an ergonomic API that is nearly identical to working with plain JavaScript arrays, while providing the reactivity needed for your application's UI to update automatically.
+
+```javascript
+import { TrackedArray } from 'tracked-built-ins';
+
+// In a component or class
+class MyComponent {
+  myArray = new TrackedArray([1, 2, 3]);
+
+  addItem() {
+    // pushObject -> push
+    this.myArray.push(4);
+  }
+
+  removeItem() {
+    // removeAt -> splice
+    this.myArray.splice(1, 1);
+  }
+
+  clearItems() {
+    // clear -> set length to 0
+    this.myArray.length = 0;
+  }
+}
+```
+
+This pattern applies to all mutation methods. Here is a brief mapping for the most common methods:
+
+| `@ember/array` Method | Native `TrackedArray` Method |
+|-----------------------|------------------------------|
+| `pushObject(s)`       | `push` / `...` (spread)      |
+| `popObject()`         | `pop`                        |
+| `shiftObject()`       | `shift`                      |
+| `unshiftObject(s)`    | `unshift`                    |
+| `insertAt(idx, obj)`  | `splice(idx, 0, obj)`        |
+| `removeAt(idx, len)`  | `splice(idx, len)`           |
+| `clear()`             | `length = 0`                 |
+| `replace()`           | `splice`                     |
+
+### Handling Uniqueness and Specific Objects
+
+For methods like `addObject` and `removeObject` that deal with specific object instances or uniqueness, you need a bit more logic.
+
+```javascript
+// In your component class with `myArray = new TrackedArray([...])`
+
+// removeObject replacement
+removeItem(item) {
+  const index = this.myArray.indexOf(item);
+  if (index > -1) {
+    this.myArray.splice(index, 1);
+  }
+}
+
+// addObject replacement
+addUniqueItem(item) {
+  if (!this.myArray.includes(item)) {
+    this.myArray.push(item);
+  }
+}
+```
+
+Alternatively, if you are working with a list that must be unique, consider using a `Set` or `TrackedSet` from `tracked-built-ins`, as they handle uniqueness automatically.