[React] Unit test a React Render Prop component

In this lesson, I use Enzyme and Jest to unit test a Counter Render Prop component. Writing integration tests are perfect for components that consume a Render Prop component. Likewise, unit tests are important to write for the Render Prop component itself, as you can test each individual unit that makes up the component.

import React from "react";
import Counter from "./Counter";

export default function CounterConsumer({ initial }) {
  return (
    <Counter initial={initial}>
      {({ increment, decrement, counter }) => (
        <div className="content" style={{ textAlign: "center" }}>
          <h1>{counter}</h1>
          <button className="button is-success" onClick={increment}>
            Increment
          </button>
          <button className="button is-danger" onClick={decrement}>
            Decrement
          </button>
        </div>
      )}
    </Counter>
  );
}

import React, { Component } from "react";

export default class Counter extends Component {
  state = { counter: this.props.initial || 0 };

  increment = () => this.setState(prevState => ({ counter: prevState.counter + 1 }));
  decrement = () => this.setState(prevState => ({ counter: prevState.counter - 1 }));

  render() {
    const { increment, decrement } = this;
    const { counter } = this.state;

    return this.props.children({ increment, decrement, counter });
  }
}

test:

import React from "react";
import ReactDOM from "react-dom";
import toJSON from "enzyme-to-json";
import { mount } from "enzyme";
import "./enzymeSetup";
import Counter from "./Counter";

it("Calls the Render Prop function", () => {
  let renderFn = jest.fn().mockReturnValue(null);
  const wrapper = mount(<Counter>{renderFn}</Counter>);

  expect(renderFn.mock.calls.length).toBe(1);
  expect(wrapper.state("counter")).toBe(0);
});

it("Accepts an initial value", () => {
  let renderFn = jest.fn().mockReturnValue(null);
  const wrapper = mount(<Counter initial={2012}>{renderFn}</Counter>);

  expect(wrapper.state("counter")).toBe(2012);
});

it("Increments", () => {
  let renderFn = jest.fn().mockReturnValue(null);
  const wrapper = mount(<Counter>{renderFn}</Counter>);

  wrapper.instance().increment();

  expect(wrapper.state("counter")).toBe(1);
});

it("Decrements", () => {
  let renderFn = jest.fn().mockReturnValue(null);
  const wrapper = mount(<Counter>{renderFn}</Counter>);

  wrapper.instance().decrement();

  expect(wrapper.state("counter")).toBe(-1);
});