Implementing promise in react

Question

I am trying to implement simple promise is react.

below is my code:

import React, { Component } from "react";

class App extends Component {
  constructor(props) {
    super(props);
    this.state = {
      name: '',
    };
  }
  componentWillMount() {
    var promise = new Promise( (resolve, reject) => {
   
     let name = 'Shubham'
   
     if (name === 'Shubham') {
      resolve("Promise resolved successfully");
     }
     else {
      reject(Error("Promise rejected"));
     }
    });
   
    promise.then( result => {
     this.setState({name: result});
    }, function(error) {
     this.setState({name: error});
    });
   }
   render() {
    return (
     <div className="App">
      <h1>Hello World</h1>
      <h2>{this.state.name}</h2>
     </div>
    );
   }
}
export default App;

This works as long as the name matches. If I change let name = 'Shubham' to let name = 'Shaurya' then it is giving me error Unhandled Rejection (TypeError): Cannot read property 'setState' of undefined. I am not able to understand what error am I doing.

Does this answer your question? How to access the correct `this` inside a callback? — jonrsharpe
– jonrsharpe, Commented Nov 12, 2020 at 7:35

Dennis Vash · Accepted Answer · 2020-11-12 07:37:18Z

1

In your promise error handler, this refers to function instance and not to the class component's. Change it to an arrow function:

promise.then( result => {
    ...
    }, (error) => {
     this.setState({name: error});
    })

edited Nov 12, 2020 at 7:37

answered Nov 12, 2020 at 7:34

Dennis Vash

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3 Comments

pratteek shaurya Over a year ago

I am getting error:

Error: Objects are not valid as a React child (found: Error: Promise rejected). If you meant to render a collection of children, use an array instead.

Dennis Vash Over a year ago

Well because error is an object, and you render this.state.name. What you trying to achieve in this.setState({name: error})? You need to render a string in this case like this.setState({name: 'Error'})

Dennis Vash Over a year ago

@pratteekshaurya Try accepting/upvoting usefull answers

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