Your example is a bit incomplete. Anyways, here is how I would do that if the returned data was an array https://codesandbox.io/s/mutable-leftpad-o3v30?file=/src/App.js

If you can use hooks then the simplest solution is useState

import {useState} from 'react'  

declare

const [newState, setNewState] = useState()

then you try

 fetch("https://flask-api-test1.herokuapp.com/predict", {
      method: "post",
      body: formData,
    })
      .then((res) => res.json())
      .then((result) => {
        setNewState(result)
        console.log(JSON.stringify(result));
      })
      .catch((err) => {
        console.log(err);
      });

and you can use ready-made 'newState' for div in jsx like

<div>
 {newState?.keyDataYouNeed}
</div>

never do this.state and assign values directly, as its immutable, what you can do is ,

fetch("https://flask-api-test1.herokuapp.com/predict", {
  method: "post",
  body: formData,
})
  .then((res) => res.json())
  .then((result) => {
    this.setState({data:result});
    console.log(result);
  })
  .catch((err) => {
    console.log(err);
  });