Not able to add chat message using Sockjs in react native - reactjs

Question

I am facing issue in adding chat message in my Flatlist because I am not able to call any function inside stompClient.subscribe ()
My code is as following :
dummy='dummyurl';
sock = new SockJS(this.dummy);
componentDidMount() {
var that = this;
this.sock.onConnect =(e) =>{
console.log("connected")
}
this.sock.onopen = function() {
console.log('open');
var dummy2='dummyurl';
var sock2 = new SockJS(dummy2);
let stompClient = Stomp.over(sock2);
stompClient.heartbeat.outgoing = 20000;
stompClient.heartbeat.incoming = 0;
stompClient.connect({}, (frame) => {
stompClient.subscribe('xyz/getChat', (messageOutput) =>{
var mymessage =JSON.parse(messageOutput.body).message;
this.state = {
incomingchatMessage: "",
chatMessages:[]
}
that.setState({incomingchatMessage:mymessage}) //getting issue setState not found
console.log(this.state)
});
});
};
this.sock.onmessage =(e) =>{
console.log('message', e);
alert("on message calledj")
};
this.sock.onmessage = evt => {
console.log("erve");
}
}
In this code I am able to get net message inside new_message variable but not able to update state value here .
Any solution of this condition .