find-consecutive-runs.js

var array = [1,2,3,5,10,9,8,9,10,11,7];

function findConsecutiveRuns(array){
  var consecutiveRuns = new Array();
  for (index=0;index<array.length;index++){
    var one = parseInt(array[index]);
    var two = parseInt(array[index+1]);
    var three = parseInt(array[index+2]);
    if (one+1==two && two + 1 == three){
      // consecutive increasing: 1,2,3
      consecutiveRuns.push(index);
    }
    if (one-1==two && two-1==three){
      // consecutive decreasing: 3,2,1
      consecutiveRuns.push(index);
    }  
  }
  if(consecutiveRuns.length){
    return consecutiveRuns;
  } else {
    return null;
  }
}

findConsecutiveRuns(array);