Workaround to test a custom hook API (https://codesandbox.io/s/jvo251qqwy)

index.test.js

import React, { useState, useEffect } from "react";
import { act } from "react-dom/test-utils";
import ShallowRenderer from "react-test-renderer/shallow";

import Enzyme, { shallow } from "enzyme";
import Adapter from "enzyme-adapter-react-16";

Enzyme.configure({ adapter: new Adapter() });

// Custom Hook API being subject to testing
import useCustomHook from "shared/hooks/useCustomHook";

const HookAPITestHelper = ({
  hook,
  hookProps: props,
  method,
  methodArgs: args
}) => {
  const [firstRender, setFirstRender] = useState(true);
  const API = hook(props);

  useEffect(() => {
    if (firstRender) {
      setFirstRender(false);
    }
  });

  if (firstRender) {
    if (method && API[method]) {
      API[method](args);
    }
    setFirstRender(false);
  } else {
    return <div data-test="hook-api" {...API} />;
  }
};

describe("useCustomHook", () => {
  let container;

  beforeEach(() => {
    container = document.createElement("div");
    document.body.appendChild(container);
  });

  afterEach(() => {
    document.body.removeChild(container);
    container = null;
  });

  it("onClick should set custom variable to true", () => {
    const renderer = new ShallowRenderer();

    act(() => {
      renderer.render(
        <HookAPITestHelper method="onClick" hook={useCustomHook} />
      );
    }, container);

    const output = renderer.getRenderOutput();
    const api = shallow(output);

    expect(api.props().variable).toBe(true);
  });
});

useCustomHook.js

import React, { useState } from "react";

export default function useCustomHook(props) {
  const [variable, setCustomVariable] = useState(false);

  function onClick(e) {
    setCustomVariable(true);
  }

  return {
    // Methods
    onClick,

    // State Variables
    variable
  };
}