javascript - undefined in variable using react componentDidMount - Stack Overflow

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I have no clue why this.role is undefined in render.

export default class Dashboard extends Component {

  ponentDidMount() {
    this.role = window.localStorage.getItem('role')
    console.log('role', this.role) //return admin
  }

  render(){
    console.log('role', this.role) //return undefined
    return(
      <div>
        Component
      </div>
    )
  }
}

I checked the localStorage of my app and it has value.

I have no clue why this.role is undefined in render.

export default class Dashboard extends Component {

  ponentDidMount() {
    this.role = window.localStorage.getItem('role')
    console.log('role', this.role) //return admin
  }

  render(){
    console.log('role', this.role) //return undefined
    return(
      <div>
        Component
      </div>
    )
  }
}

I checked the localStorage of my app and it has value.

• javascript
• reactjs

Share Improve this question Follow asked Jan 8, 2018 at 13:51 Jenny MokJenny Mok 2,80499 gold badges3333 silver badges6262 bronze badges

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3 Answers 3

Sorted by: Reset to default 3

what happens is that at the initial render, render() method is called (before ponentDidMount() is called), so it shows 'undefined'. changing the value of 'this.role' won't re-render the page.

You will have to use state for this. Below code should work I believe.

export default class Dashboard extends Component {
constructor(){
  super()
  this.state = {
    role : undefined
    }
} 

ponentDidMount() {
    this.setState({role: window.localStorage.getItem('role')})
    console.log('role', this.role) //return admin
  }

  render(){
    console.log('role', this.state.role) //return undefined
    return(
      <div>
        Component
      </div>
    )
  }
}

It's returning undefined because you're setting this.role after the ponent is mount (ponentDidMount). So the first render doesn't have this.role.

After ponentDidMount is run you're not changing the state and the render is not running again (and therefore not getting the new info).

Try with ponentWillMount instead, it should probably work.

Here's the React Lifecycle documentation.

Edit: Added code.

export default class Dashboard extends Component {

  ponentWillMount() {
    this.role = window.localStorage.getItem('role')
    console.log('role', this.role) // return admin
  }

  render(){
    console.log('role', this.role) // now returning admin because this.role is set before the 1st render
    return(
      <div>
        Component
      </div>
    )
  }
}

As other users have pointed out, you can also use setState instead and it would also work (In that case, when the state changes the render is run again and your role is displayed accordingly).

You see undefined in the view because by the time the ponent has rendered there was nothing in role because ponentDidMount is called after the initial render. Moreover, the ponent doesn't rerender after you have set role value from localStorage because it is not on the state. If you place role on the state and do this:

ponentDidMount() {
  this.setState({ role: window.localStorage.getItem('role')});
}

render(){
  console.log('role', this.state.role)
  return(
      <div>
        Component
      </div>
    )
}

then you will be able to see value of role in the view, but it will cause extra rerender of the ponent since you will change its state, according to react docs about ponentDidMount:

Calling setState() in this method will trigger an extra rendering, but it will happen before the browser updates the screen.

You can read more about ponentDidMount here.

---

Update:

In your case you don't have to put role on the state, but then you can fetch its value from the constructor:

constructor(props) {
  super(props);
  this.role = window.localStorage.getItem('role');
}

and it will be available in the view.

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