elision.js

var elide = function(arr) {
  // assume arr is sorted, or put it through a sort right here.
  var tokens = []; // this'll be what gets output at the end.
  var startOfRun = arr[0]; // pretend we have a stack. This is the first value pushed onto the stack.
  var currentEndOfRun = startOfRun; // this is the top of our pretend stack.
  var coalesce = function(start, end){ return "" + ((start != end) ? ""+start+"-"+end : start); }
  
  // Consider each number in the array and it's relationship to the current run on the stack.
  for (var i=1; i < arr.length; i++) {
    // If the nextNumber is adjacent to the value at the top of the stack
    var nextNumber = arr[i];
    if (nextNumber == currentEndOfRun + 1) {
      // keep running! and push it onto the top of our pretend stack.
      currentEndOfRun = arr[i];
    } else {
      // otherwise, if the nextNumber isn't adjacent
      // the previous run is complete. Coalesce the stack & push it into the tokens.
      tokens.push(coalesce(startOfRun,currentEndOfRun));
      // clear the stack & push the nextNumber as the new start.
      startOfRun = nextNumber;
      currentEndOfRun = startOfRun;
    }
  }
  tokens.push(coalesce(startOfRun,currentEndOfRun)); // flush the final token.
  return tokens.join(', '); // join the tokens for the fun of it.
};

console.log(elide([1,2,3,4,6,15,16,17,18,19,20,43])); // should print `1-4, 6, 15-20, 42`