jest.replaceProperty(object, propertyKey, value)

The property must already exisy on the object. All properties could be restored to the original value by calling jest.restoreAllMocks on afterEach method.

test("isLocalhost return true when HOSTNAME is localhost", () => {
	jest.replaceProperty(process, "env", { HOSTNAME:"localhost" });
	expect(utils.isLocalhost()).toBe(true);
});