Skip implementation in Jest - unit-testing

Question

Currently I have the following piece of code:
function handleConnection(socket: Socket): void {
info(`Connection started with socketId: ${socket.id}`)
socket.on("joinRoom", (request: string) => handleJoinRoom(socket, request));
socket.on("shareData", (request: IShareDataRequest) => handleShareData(socket, request));
socket.on("disconnect", () => handleDisconnect(socket));
}
I want to write a test for each and every events like joinRoom, shareData and disconnect. To isolae the tests I want to only test the second socket.on("shareData", () => ...) call and skip the first socket.on("joinRoom", () => ...) call. I've tried with multiple mockImplementationOnce methods with no success.
The test I wrote:
it('should emit data to room', () => {
const listenMock = listen();
const socketMock = {
on: jest.fn()
.mockImplementationOnce(() => null)
.mockImplementationOnce((event: string, callback: Function) => callback({ roomId: "abcdefg" })),
to: jest.fn().mockReturnValue({
emit: jest.fn()
})
}
jest.spyOn(listenMock, "on").mockImplementationOnce((event: string, callback: Function) => callback(socketMock))
startSocket();
expect(socketMock.to).toHaveBeenCalledWith("abcdefg");
expect(socketMock.to().emit).toHaveBeenCalledWith("receiveData", expect.any(Object));
})
ShareData function:
function handleShareData(socket: Socket, request: IShareDataRequest): void {
socket.to(request.roomId).emit("receiveData", request);
}
I would really appreciate if anyone could help me out with this.

Answer 1

You can try the following approach:
// define the mockSocket
const mockSocket = {
// without any implementation
on: jest.fn()
};
describe("connection handler", () => {
// I personally like separating the test setup
// in beforeAll blocks
beforeAll(() => {
handleConnection(mockSocket);
});
// you can write assertions that .on
// should have been called for each event
// with a callback
describe.each(["joinRoom", "shareData", "disconnect"])(
"for event %s",
event => {
it("should attach joinRoom handlers", () => {
expect(mockSocket.on.mock.calls).toEqual(
expect.arrayContaining([[event, expect.any(Function)]])
);
});
}
);
describe("joinRoom handler", () => {
beforeAll(() => {
// jest mock functions keep the calls internally
// and you can use them to find the call with
// the event that you need and retrieve the callback
const [_, joinRoomHandler] = mockSocket.on.mock.calls.find(
([eventName]) => eventName === "joinRoom"
);
// and then call it
joinRoomHandler("mockRequestString");
});
it("should handle the event properly", () => {
// your joinRoom handler assertions would go here
});
});
});