Fastest way to get count of unique elements in javascript array

Question

I need to find the number of unique elements in an array.

var myArray = [ 10, 10, 20, 20, 30, 30, 40, 40, 40, 40, 50, 50, 50, 50, 60 ];

I want count = 6 (number of unique elements in array)

And, is there a way to do this without iterating through the array? (I assume that would be the fastest way?).

ANSWER: I used the .filter method as shown below. My actual array element is much more complex. I ended up iterating through my array and created a new array. Each element in the new array was a .substr of the old element. Then, the .filter method below worked great. THANKS to everyone!!

Answer 1

You need to keep a set of known values, and an auxilliary count. You can use .reduce():

var count = myArray.reduce(function(values, v) {
  if (!values.set[v]) {
    values.set[v] = 1;
    values.count++;
  }
  return values;
}, { set: {}, count: 0 }).count;

Starting with an empty set of values and a count of zero, this looks at each element to see whether it's been added to the set. If it hasn't, it's added, and the count is incremented.

Answer 2

ES6 offers a single line solution:

new Set(myArray).size