JavaScript get previous work day

getPrevWorkDay.js

function getPrevWorkDay (t) {
  if(!t) t = new Date();
  if( !(t instanceof Date) ) t = new Date(t);
  var prevDay = new Date( t.getTime());
  do {
    prevDay = new Date( prevDay.getTime() - 86400000 );
  } while( -1 !== [0, 6].indexOf(prevDay.getDay()) );
  return prevDay;
}