fix(reactExampleIntro): Missing initialState for useReducer in react example-intro (testing-library#538)

Closes testing-library#537

Author: MatanBobi

docs/react-testing-library/example-intro.md

@@ -174,6 +174,11 @@ fetch.js
 import React, { useState, useReducer } from 'react'
 import axios from 'axios'
 
+const initialState = {
+ error: null,
+ greeting: null,
+}
+
 function greetingReducer(state, action) {
  switch (action.type) {
  case 'SUCCESS': {
@@ -185,7 +190,7 @@ function greetingReducer(state, action) {
  case 'ERROR': {
  return {
  error: action.error,
- greeting: null
+ greeting: null,
  }
  }
  default: {
@@ -195,18 +200,22 @@ function greetingReducer(state, action) {
 }
 
 export default function Fetch({ url }) {
- const [{ error, greeting }, dispatch] = useReducer(greetingReducer)
+ const [{ error, greeting }, dispatch] = useReducer(
+ greetingReducer,
+ initialState
+ )
  const [buttonClicked, setButtonClicked] = useState(false)
 
  const fetchGreeting = async () => {
- axios.get(url)
- .then((response) => {
+ axios
+ .get(url)
+ .then(response => {
  const { data } = response
  const { greeting } = data
  dispatch({ type: 'SUCCESS', greeting })
  setButtonClicked(true)
  })
- .catch((error) => {
+ .catch(error => {
  dispatch({ type: 'ERROR', error })
  })
  }