How to mock navigator.clipboard.writeText() in Jest?

Jest tests are running in JSdom environment and not all of the properties are defined, but so you should define the function before spying on it.

Here is an example:

const writeText = jest.fn()

Object.assign(navigator, {
  clipboard: {
    writeText,
  },
});

describe("Clipboard", () => {
  describe("writeText", () => {
    beforeAll(() => {
      navigator.clipboard.writeText.mockResolvedValue(undefined)
      // or if needed
      // navigator.clipboard.writeText.mockRejectedValue(new Error()) 
      yourImplementationThatWouldInvokeClipboardWriteText();
    });
    it("should call clipboard.writeText", () => {
      expect(navigator.clipboard.writeText).toHaveBeenCalledWith("zxc");
    });
  });
});

Edit: you can also use Object.defineProperty, but it accepts descriptors object as third parameter

Object.defineProperty(navigator, "clipboard", {
  value: {
    writeText: async () => {},
  },
});