code: function incrementingRunsFrom(list) {
          list = list.concat([Infinity]);// use Infinity to ensure any nontrivial (length >= 2) run ends before the end of the loop
          var runs = [];
          var start = null;
          var prev = null;
          for (var i = 0; i < list.length; i++) {
              var current = list[i];
              if (start === null) {
                  // first element starts a trivial run
                  start = current;
              } else if (prev + 1 !== current) {
                  // run ended
                  if (start !== prev) {
                      // run is nontrivial
                      runs.push([start, prev]);
                  }
                  // start new run
                  start = current;
              }
              // else: the run continues
  
              prev = current;
          }
          return runs;
      }
docs: Given a sorted array of integers, this finds all contiguous runs where each item is incremented by 1 from the next.
  For example:
  [0, 2, 3, 5] has one such run: [2, 3]
  [0, 2, 3, 4, 6, 8, 9, 11] has two such runs: [2, 3, 4], [8, 9]
  [0, 2, 4] has no runs.
  @param {integer[]} list Sorted array of integers
  @returns {Array.<Array.<integer>>} Array of pairs of start and end values of runs