test(mergeWithKey): add tests to mergeWithKey

packages/utils/src/mergeWithKey.spec.ts

@@ -0,0 +1,34 @@
+import { map } from "rxjs/operators"
+import { TestScheduler } from "rxjs/testing"
+import { mergeWithKey } from "./mergeWithKey"
+
+const scheduler = () =>
+  new TestScheduler((actual, expected) => {
+    expect(actual).toEqual(expected)
+  })
+
+describe("mergeWithKey", () => {
+  it("emits the key of the stream that emitted the value", () => {
+    scheduler().run(({ expectObservable, cold }) => {
+      const sourceA = cold("a---b---|");
+      const sourceB = cold("-1--2----3|")
+      const expected = "    mn--(op)-q|"
+
+      const result = mergeWithKey({
+        strings: sourceA,
+        numbers: sourceB
+      }).pipe(
+        map(({ type, payload }) => `${type}:${payload}`),
+      )
+
+      expectObservable(result).toBe(expected, {
+        m: "strings:a",
+        n: "numbers:1",
+        o: "strings:b",
+        p: "numbers:2",
+        q: "numbers:3",
+      })
+    })
+  })
+})
+

packages/utils/src/mergeWithKey.ts

@@ -1,6 +1,12 @@
 import { merge, Observable, ObservableInput, from, SchedulerLike } from "rxjs"
 import { map } from "rxjs/operators"
 
+/**
+ * Emits the values from all the streams of the provided object, in a result
+ * which provides the key of the stream of that emission.
+ * 
+ * @param input object of streams
+ */
 export const mergeWithKey: <
   O extends { [P in keyof any]: ObservableInput<any> },
   OT extends {