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;
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 .
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