socket.send outside of io.sockets.on( )

I think that this will successfully solve your problem

io.sockets.emit('hello')

Improving previous answer from @Nyxynyx

io.sockets.emit('hello')

It is equivalent to:

io.of('/').emit('hello');

If you need a different route, change the of function parameter.

Documentation: https://socket.io/docs/v3/server-api/index.html#server-sockets