This is robust code which determines if a string exists in a JavaScript array. It returns true or false. If either parameter is invalid it throws an error.

stringInArray = function(s, a) {
  if ((s === undefined) || (a === undefined)) {
    throw "a or s is undefined";
  }
  if (typeof s !== 'string') {
    throw "'" + s + "' is not a string";
  }
  if (a.constructor !== Array) {
    throw "'" + a + "' is not an Array";
  }
  if (s.length == 0) {
    throw "'" + s + "' is 0 length";
  }
  return (a.indexOf(s) > -1);
}