Add support for obs. arrays on keys. Fixes #1600

src/api/object-api.ts

@@ -12,6 +12,7 @@ import { getAdministration } from "../types/type-utils"
 import { ObservableValue } from "../types/observablevalue"
 
 export function keys<K>(map: ObservableMap<K, any>): ReadonlyArray<K>
+export function keys<T>(ar: IObservableArray<T>): ReadonlyArray<number>
 export function keys<T extends Object>(obj: T): ReadonlyArray<string>
 export function keys(obj: any): any {
     if (isObservableObject(obj)) {
@@ -20,9 +21,12 @@ export function keys(obj: any): any {
     if (isObservableMap(obj)) {
         return (obj as any)._keys.slice()
     }
+    if (isObservableArray(obj)) {
+        return obj.map((_, index) => index)
+    }
     return fail(
         process.env.NODE_ENV !== "production" &&
-            "'keys()' can only be used on observable objects and maps"
+            "'keys()' can only be used on observable objects, arrays and maps"
     )
 }
 

test/base/object-api.js

@@ -223,6 +223,22 @@ test("array - set, remove, entries are reactive", () => {
     ])
 })
 
+test("array - set, remove, keys are reactive", () => {
+    const todos = observable.array()
+    const snapshots = []
+
+    reaction(() => keys(todos), keys => snapshots.push(keys))
+
+    set(todos, 0, 2)
+    set(todos, "1", 4)
+    set(todos, 3, 4)
+    set(todos, 1, 3)
+    remove(todos, 2)
+    remove(todos, "0")
+
+    expect(snapshots).toEqual([[0], [0, 1], [0, 1, 2, 3], [0, 1, 2, 3], [0, 1, 2], [0, 1]])
+})
+
 test("observe & intercept", () => {
     let events = []
     const todos = observable(