fastest way to detect if duplicate entry exists in javascript array?

Question

var arr = [\'test0\',\'test2\',\'test0\'];

Like the above,there are two identical entries with value \"test0\",how to check it most efficiently?

Answer 1

Sorting is O(n log n) and not O(n). Building a hash map is O(n). It costs more memory than an in-place sort but you asked for the "fastest." (I'm positive this can be optimized but it is optimal up to a constant factor.)

function hasDuplicate(arr) {
  var hash = {};
  var hasDuplicate = false;
   arr.forEach(function(val) {
     if (hash[val]) {
       hasDuplicate = true;
       return;
     }
     hash[val] = true;
  });
  return hasDuplicate;
}