在componentDidMount（）上传递要更新的参数的函数内部的设置状态做出反应

Question

I need to update component state object elements from function inside componentDidMount(). Ideally I want to be able to pass the element I need to update and the value I want to update it with. I do understand "this" is undefined and I have seen solutions with arrow functions but I can't seem to get it to work.

What would be the best approach to update state from function inside componentDidMount() passing element and value to be updated?

class App extends Component {

  constructor(props) {
    super(props);
    this.state = {
      obj: {
        itemone: "on",
        itemtwo: "off"
      }
    };
  }

  // updateState(item, valStr) {
  //   this.setState({
  //       obj: {
  //         item: valStr
  //       }
  //   });
  // };

  componentDidMount() { 
      //.......
      websocket.onmessage = function (event) {
        //this.updateState(itemone "off");
        this.setState({ //undefined
            obj: {
              itemone: "off"
            }
        });
      };
  }

  render() {

    return (
      <div className="App">
        ...
      </div>
    );
  }
}

export default App;

Answer 1

try this

websocket.onmessage = (function (event) {
        //this.updateState(itemone "off");
        this.setState({
            obj: {
              itemone: "off"
            }
        });
}).bind(this);

Answer 2

Given your inability to use arrow functions, you can circumvent the this coming undefined by:

componentDidMount() { 

// Define a variable pointing to the `this` of the class
const newThis = this
websocket.onmessage = function (event) {
   // Use that this to call the setState
   newThis.setState(prevState => {
      const newObj = prevState.obj
      newObj.itemone = "off"
      return { obj: newObj}
   })
  }
}

Answer 3

You can try the arrow function

websocket.onmessage = (event) => {
    //this.updateState(itemone "off");
    this.setState({ //undefined
        obj: {
          itemone: "off"
        }
    });
  };