Error - this.socket.onMessage is not a function

[Wathsara]

var initialize = "null";
const socket:rpc.IWebSocket = new WebSocket('ws://localhost:8080/mypath');
const reader = new rpc.WebSocketMessageReader(socket);
const writer = new rpc.WebSocketMessageWriter(socket);
const logger = new rpc.ConsoleLogger();
const connection = rpc.createMessageConnection(reader, writer, logger);
const notification = new rpc.NotificationType<string, void>('onInitialize');
connection.onNotification(notification, (param: any) => {
	initialize=param; // This prints Hello World
});
connection.listen();

When I tried the above code which is in the readme file it gives an error like before

Request textDocument/completion failed with message: this.socket.onMessage is not a function