var a = ["CS348", "CS353", "CS381", "CS422", "CS448", "CS490-ES0", "CS490-DSO"];

var b = ["CS348", "CS352", "CS353", "CS354", "CS381", "CS422", "CS448", "CS456", "CS473", "CS490-DSO", "CS490-ES0"];

​

var c = a.concat(b).filter(function(el) {

  return a.indexOf(el) > -1 && b.indexOf(el) > -1;

});

​

​

console.log(c);

​

console.log(a.length);

console.log(b.length);

console.log(c.length);