Support HTTP headers with CORS-method (#2957)

Adds support for sending and receiving HTTP-headers when using the
CORS-method.

This change is required for the Yr weather-provider introduced in
https://github.com/MichMich/MagicMirror/pull/2948.

To make it easier to add unit tests I moved the server-functions into a
separate file.

tests/unit/functions/server_functions_spec.js

@@ -0,0 +1,149 @@
+const { cors } = require("../../../js/server_functions");
+
+describe("server_functions tests", () => {
+	describe("The cors method", () => {
+		let fetchResponse;
+		let fetchResponseHeadersGet;
+		let fetchResponseHeadersText;
+		let corsResponse;
+		let request;
+
+		jest.mock("node-fetch");
+		let nodefetch = require("node-fetch");
+		let fetchMock;
+
+		beforeEach(() => {
+			nodefetch.mockReset();
+
+			fetchResponseHeadersGet = jest.fn(() => {});
+			fetchResponseHeadersText = jest.fn(() => {});
+			fetchResponse = {
+				headers: {
+					get: fetchResponseHeadersGet
+				},
+				text: fetchResponseHeadersText
+			};
+			jest.mock("node-fetch", () => jest.fn());
+			nodefetch.mockImplementation(() => fetchResponse);
+
+			fetchMock = nodefetch;
+
+			corsResponse = {
+				set: jest.fn(() => {}),
+				send: jest.fn(() => {})
+			};
+
+			request = {
+				url: `/cors?url=www.test.com`
+			};
+		});
+
+		test("Calls correct URL once", async () => {
+			const urlToCall = "http://www.test.com/path?param1=value1";
+			request.url = `/cors?url=${urlToCall}`;
+
+			await cors(request, corsResponse);
+
+			expect(fetchMock.mock.calls.length).toBe(1);
+			expect(fetchMock.mock.calls[0][0]).toBe(urlToCall);
+		});
+
+		test("Forewards Content-Type if json", async () => {
+			fetchResponseHeadersGet.mockImplementation(() => "json");
+
+			await cors(request, corsResponse);
+
+			expect(fetchResponseHeadersGet.mock.calls.length).toBe(1);
+			expect(fetchResponseHeadersGet.mock.calls[0][0]).toBe("Content-Type");
+
+			expect(corsResponse.set.mock.calls.length).toBe(1);
+			expect(corsResponse.set.mock.calls[0][0]).toBe("Content-Type");
+			expect(corsResponse.set.mock.calls[0][1]).toBe("json");
+		});
+
+		test("Forewards Content-Type if xml", async () => {
+			fetchResponseHeadersGet.mockImplementation(() => "xml");
+
+			await cors(request, corsResponse);
+
+			expect(fetchResponseHeadersGet.mock.calls.length).toBe(1);
+			expect(fetchResponseHeadersGet.mock.calls[0][0]).toBe("Content-Type");
+
+			expect(corsResponse.set.mock.calls.length).toBe(1);
+			expect(corsResponse.set.mock.calls[0][0]).toBe("Content-Type");
+			expect(corsResponse.set.mock.calls[0][1]).toBe("xml");
+		});
+
+		test("Sends correct data from response", async () => {
+			const responseData = "some data";
+			fetchResponseHeadersText.mockImplementation(() => responseData);
+
+			let sentData;
+			corsResponse.send = jest.fn((input) => {
+				sentData = input;
+			});
+
+			await cors(request, corsResponse);
+
+			expect(fetchResponseHeadersText.mock.calls.length).toBe(1);
+			expect(sentData).toBe(responseData);
+		});
+
+		test("Sends error data from response", async () => {
+			const error = new Error("error data");
+			fetchResponseHeadersText.mockImplementation(() => {
+				throw error;
+			});
+
+			let sentData;
+			corsResponse.send = jest.fn((input) => {
+				sentData = input;
+			});
+
+			await cors(request, corsResponse);
+
+			expect(fetchResponseHeadersText.mock.calls.length).toBe(1);
+			expect(sentData).toBe(error);
+		});
+
+		test("Fetches with user agent by default", async () => {
+			await cors(request, corsResponse);
+
+			expect(fetchMock.mock.calls.length).toBe(1);
+			expect(fetchMock.mock.calls[0][1]).toHaveProperty("headers");
+			expect(fetchMock.mock.calls[0][1].headers).toHaveProperty("User-Agent");
+		});
+
+		test("Fetches with specified headers", async () => {
+			const headersParam = "sendheaders=header1:value1,header2:value2";
+			const urlParam = "http://www.test.com/path?param1=value1";
+			request.url = `/cors?${headersParam}&url=${urlParam}`;
+
+			await cors(request, corsResponse);
+
+			expect(fetchMock.mock.calls.length).toBe(1);
+			expect(fetchMock.mock.calls[0][1]).toHaveProperty("headers");
+			expect(fetchMock.mock.calls[0][1].headers).toHaveProperty("header1", "value1");
+			expect(fetchMock.mock.calls[0][1].headers).toHaveProperty("header2", "value2");
+		});
+
+		test("Sends specified headers", async () => {
+			fetchResponseHeadersGet.mockImplementation((input) => input.replace("header", "value"));
+
+			const expectedheaders = "expectedheaders=header1,header2";
+			const urlParam = "http://www.test.com/path?param1=value1";
+			request.url = `/cors?${expectedheaders}&url=${urlParam}`;
+
+			await cors(request, corsResponse);
+
+			expect(fetchMock.mock.calls.length).toBe(1);
+			expect(fetchMock.mock.calls[0][1]).toHaveProperty("headers");
+			expect(corsResponse.set.mock.calls.length).toBe(3);
+			expect(corsResponse.set.mock.calls[0][0]).toBe("Content-Type");
+			expect(corsResponse.set.mock.calls[1][0]).toBe("header1");
+			expect(corsResponse.set.mock.calls[1][1]).toBe("value1");
+			expect(corsResponse.set.mock.calls[2][0]).toBe("header2");
+			expect(corsResponse.set.mock.calls[2][1]).toBe("value2");
+		});
+	});
+});