This one looks to be trival:

    this.sysProps = function() {
      for (var i in jvm.sysProps) {
         writeln(i + ' = ' + jvm.sysProps[i]);
      }
    }
    registerCommand("sysprops", "sysprops", "sysProps");

jvm.sysProps[] is a cache initialized on the java side by the following:

    private synchronized JSMap getSysProps() {
        if (sysPropsCache == null) {
            Properties props = vm.getSystemProperties();
            Map map = new HashMap();
            if (props != null) {
                Enumeration e = props.propertyNames();
                while (e.hasMoreElements()) {
                    String key = (String) e.nextElement();
                    map.put(key, props.getProperty(key));
                }
            }
            sysPropsCache = factory.newJSMap(map);
        }
        return sysPropsCache;
    }

We don't need to keep a cache, so the implementation can be a similar loop to what is in getSysProps(), but just printing the key+value rather than caching it.

Output from old javascript sysprops command looks like the following:

awt.toolkit = sun.awt.X11.XToolkit 
file.encoding.pkg = sun.io 
java.specification.version = 1.8 
...