This is literally impossible. Jade is rendered into static HTML before being served to the client. Whatever variables you put in are parsed at the time of rendering only.
You're gonna have to go with javascript to do any run-time DOM manipulations like that.
script. var J1 = 0; var socket = io.connect('http://localhost:3000'); socket.on('positionJ1', function(positionJ1) { J1 = positionJ1; document.getElementById('position').innerHTML= J1; }) p Le joueur 1 est en case span#position