How to get unique values in an array

Question

How can I get a list of unique values in an array? Do I always have to use a second array or is there something similar to java's hashmap in JavaScript?

I am going to be using JavaScript and jQuery only. No additional libraries can be used.

Answer 1

Since I went on about it in the comments for @Rocket's answer, I may as well provide an example that uses no libraries. This requires two new prototype functions, contains and unique

Array.prototype.contains = function(v) {
  for (var i = 0; i < this.length; i++) {
    if (this[i] === v) return true;
  }
  return false;
};

Array.prototype.unique = function() {
  var arr = [];
  for (var i = 0; i < this.length; i++) {
    if (!arr.contains(this[i])) {
      arr.push(this[i]);
    }
  }
  return arr;
}

var duplicates = [1, 3, 4, 2, 1, 2, 3, 8];
var uniques = duplicates.unique(); // result = [1,3,4,2,8]

console.log(uniques);

For more reliability, you can replace contains with MDN's indexOf shim and check if each element's indexOf is equal to -1: documentation