this.web3 = new Web3(new Web3.providers.WebsocketProvider('ws://127.0.0.1:8546'));
...

let postData = {
            ttl: 2000,
            topic: defaultTopic,
            powTarget: 0.2,
            powTime: 20,
            payload: encodeToHex(JSON.stringify({
                _id: this.state.username,
                text: messages[0].text,
                name: this.state.username,
            })),
            symKeyID: some symKey,
            sig: some asymKeyId,
        };

this.web3.shh.post(postData);

let options = {
            topics: [defaultTopic],
            symKeyID: some symKey
        };
this.web3.shh.subscribe('messages', options)
                    .on('data', (message) => { ...