Introduce utility to find exactly one item from array

Co-authored-by: Mikko Aspiala <mikko.aspiala@gmail.com>

Signed-off-by: Janne Savolainen <janne.savolainen@live.fi>

src/common/utils/find-exactly-one/find-exactly-one.test.ts

@@ -0,0 +1,34 @@
+/**
+ * Copyright (c) OpenLens Authors. All rights reserved.
+ * Licensed under MIT License. See LICENSE in root directory for more information.
+ */
+
+import { findExactlyOne } from "./find-exactly-one";
+
+describe("find-exactly-one", () => {
+  it("when predicate matches to single item, returns the item", () => {
+    const actual = findExactlyOne((item) => item === "some-item")([
+      "some-item",
+      "some-other-item",
+    ]);
+
+    expect(actual).toBe("some-item");
+  });
+
+  it("when predicate matches to many items, throws", () => {
+    expect(() => {
+      findExactlyOne((item) => item === "some-item")([
+        "some-item",
+        "some-item",
+      ]);
+    }).toThrow("Tried to find exactly one, but found many");
+  });
+
+  it("when predicate does not match, throws", () => {
+    expect(() => {
+      findExactlyOne((item) => item === "some-item")([
+        "some-other-item",
+      ]);
+    }).toThrow("Tried to find exactly one, but didn't find any");
+  });
+});

src/common/utils/find-exactly-one/find-exactly-one.ts

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+/**
+ * Copyright (c) OpenLens Authors. All rights reserved.
+ * Licensed under MIT License. See LICENSE in root directory for more information.
+ */
+export const findExactlyOne = <T>(predicate: (item: T) => boolean) => (collection: T[]): T => {
+  const itemsFound = collection.filter(predicate);
+
+  if (!itemsFound.length) {
+    throw new Error(
+      "Tried to find exactly one, but didn't find any",
+    );
+  }
+
+  if (itemsFound.length > 1) {
+    throw new Error(
+      "Tried to find exactly one, but found many",
+    );
+  }
+
+  return itemsFound[0];
+};