Turn an array of integers (1, 2, 3, 4, 7, 9, 10) into a string of bins (1-4, 7, 9-10)

binnify.js

var binnify = function(arr) {
  var output = [];
  var bin_string = (arr) => (arr.length == 1) ? `${arr[0]}` : `${arr[0]}-${arr[arr.length - 1]}`;
  var bin_start = 0;
  for (var i = 1; i < arr.length; i++) {
    if (arr[i] != arr[i - 1] + 1) {
      output.push(bin_string(arr.slice(bin_start, i)));
      bin_start = i;
    }
  }
  output.push(bin_string(arr.slice(bin_start)));
  return output.join(', ');
};