React无法读取undefined的属性映射

Question

I am very new to react and I am trying to bring in data from a rails api but I am getting the error TypeError: Cannot read property 'map' of undefined

If i use the react dev tools I can see the state and I can see the contacts if I mess around with it in the console using $r.state.contacts Can someone help with what I have done wrong? my component looks like this:

import React from 'react';
import Contact from './Contact';

class ContactsList extends React.Component {
  constructor(props) {
    super(props)
    this.state = {}
  }

  componentDidMount() {
    return fetch('http://localhost:3000/contacts')
      .then(response => response.json())
      .then(response => {
        this.setState({
          contacts: response.contacts
        })
      })
      .catch(error => {
        console.error(error)
      })
  }

  render(){
    return(
     <ul>
        {this.state.contacts.map(contact => { return <Contact contact{contact} />})}
      </ul>
    )
  }
}

export default ContactsList;

Answer 1

Cannot read property 'map' of undefined, Why?

Because this.state is initially {} , and contacts of {} will be undefined . Important point is, componentDidMount will get called after initial rendering and it is throwing that error during first rendering.

Possible Solutions:

1- Either define the initial value of contacts as [] in state:

constructor(props) {
  super(props)
    this.state = {
       contacts: []
    }
}

2- Or put the check before using map on it:

{this.state.contacts && this.state.contacts.map(....)

For checking array, you can also use Array.isArray(this.state.contacts) .

Note: You need to assign unique key to each element inside map, check the DOC .