JavaScript: How to know if a connection with a shared worker is still alive?

This is only as reliable as beforeunload, but seems to work (tested in Firefox and Chrome). I definitely favour it over a polling solution.

// Tell the SharedWorker we're closing
addEventListener( 'beforeunload', function()
{
    port.postMessage( {command:'closing'} );
});

Then handle the cleanup of the port object in the SharedWorker.

e.ports[0].onmessage = function( e )
{
    const port = this,
    data = e.data;

    switch( data.command )
    {
        // Tab closed, remove port
        case 'closing': myConnections.splice( myConnections.indexOf( port ), 1 );
            break;
    }
}