Audit a suite

Here is a module and a suite somebody else wrote for it. Read them, decide how much of the module’s behaviour this suite actually pins down, and commit to a number. Then run the board.

The module — One or many

1		export function label(count) {
2		if (count > 1) {
3		return "many";
4		}
5		return "one";
6		}
7

The suite under review

describe("label", () => {
  test("handles a plural count", () => {
    expect(() => label(5)).not.toThrow();
  });

  test("handles a single item", () => {
    expect(() => label(1)).not.toThrow();
  });

  test("handles none", () => {
    expect(() => label(0)).not.toThrow();
  });
});

Your estimate

What share of the killable changes to that module do you think this suite would catch?

Mutation score70%

Worth asking as you read: does each test assert on a value, or on a shape, a type, or the absence of an exception? Are the inputs interesting? Does anything exercise the error path?