'TypeError: this is undefined' occurs when using members of a class from a functional component

Question

I am trying to pass data to a ReactJS component using NodeJS by making a simple NodeJS API hosted in a separate localhost (:9000). But I'm getting an error.

TypeError: this is undefined

My belief is that this error occurred due to using 'this' inside the functional component. But I cannot find a way to resolve this problem.

The code which has the error:

import React from 'react';
import ReactDOM from 'react-dom';

class News extends React.Component{


   constructor (props){
      super(props);
      this.state={apiResponse:""};

   }

   callAPI(){
      fetch("http://localhost:9000/testAPI")
      .then(res => res.text ())
      .then(res => this.setState({apiResponse: res}));
      
      
   }

   componentWillMount(){
      this.callAPI();
   }
}

function Newsexport() {
 return (
    <div class="container1">
       <h1>IT WORKS YO!</h1>
      <p>{this.state.apiResponse}</p> 
    </div>
  )
};

export default Newsexport;

The code of the simple API hosted with NodeJS (/api/routes/testAPI.js)

var express = require("express");
var router=express.Router();

router.get("/", function(req,res,next){
    res.send("API is working");

});

module.exports=router;

Answer 1

You are using this in a functional component which is wrong. Also you are setting state in one component and expecting the value in another component. Instead combine the two components like below -

 class News extends React.Component{

    constructor (props){
      super(props);
      this.state={apiResponse:""};
    }

    callAPI = () => {
      fetch("http://localhost:9000/testAPI")
        .then(res => res.text ())
        .then(res => this.setState({apiResponse: res}));
    }

    componentWillMount(){
      this.callAPI();
    }

    render() {
      return (
        <div class="container1">
          <h1>IT WORKS YO!</h1>
          <p>{this.state.apiResponse}</p> 
        </div>
      )
    }
  }
  export default News;

Let me know if this helps.