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

Loop stops when found first duplicate:

function has_duplicates(arr) {

    var x = {}, len = arr.length;
    for (var i = 0; i < len; i++) {
        if (x[arr[i]]) {
             return true;
        }
        x[arr[i]] = true;
    }
    return false;

}

Edit (fix 'toString' issue):

function has_duplicates(arr) {

    var x = {}, len = arr.length;
    for (var i = 0; i < len; i++) {
        if (x[arr[i]] === true) {
             return true;
        }
        x[arr[i]] = true;
    }
    return false;

}

this will correct for case has_duplicates(['toString']); etc..

Answer 2

If you sort the array, the duplicates are next to each other so that they are easy to find:

arr.sort();
var last = arr[0];
for (var i=1; i<arr.length; i++) {
   if (arr[i] == last) alert('Duplicate : '+last);
   last = arr[i];
}

Answer 3

This will do the job on any array and is probably about as optimized as possible for handling the general case (finding a duplicate in any possible array). For more specific cases (e.g. arrays containing only strings) you could do better than this.

function hasDuplicate(arr) {
    var i = arr.length, j, val;

    while (i--) {
     val = arr[i];
     j = i;
     while (j--) {
      if (arr[j] === val) {
       return true;
      }
     }
    }
    return false;
}