My javascript websockets seem to be buffering incoming messages

let startTime = new Date();
ws.onmessage = function (evt)
{

prevData = recivedData;
var receivedMsg = evt.data;

recivedData = JSON.parse(receivedMsg);

const endTime = new Date();
const timeDif = endTime - startTime;
startTime = endTime;
console.log(timeDif)
}

But it seems timeDif sometimes is 0, and I find this unlikely. People in other questions of mine have sad that its due to the websocket buffering incoming messages. How do I prevent this?