roll20-sheetworker-waterfall.js

var asyncWaterfall = function (tasks, cb) {
      var current = 0
    
      function done (err, args) {
        function end () {
          args = args ? [].concat(err, args) : [ err ]
          if (cb) cb.apply(undefined, args)
        }
        end();
      }
    
      function each (err) {
        var args = Array.prototype.slice.call(arguments, 1)
        if (++current >= tasks.length || err) {
          done(err, args)
        } else {
          tasks[current].apply(undefined, [].concat(args, each))
        }
      }
    
      if (tasks.length) {
        tasks[0](each)
      } else {
        done(null)
      }
    };
    
    on("change:test1", function() {
         asyncWaterfall([
            function(callback) {
                log("Waterfall One");
                getAttrs(["test1"], function(vals) {
                    log("Got attributes");
                    callback(null, vals);
                });
            },
            function(prevvals, callback) {
                log("Waterfall Two");
                setAttrs({
                    test2: parseInt(prevvals.test1, 10) + 5
                }, {}, function() {
                    log("DONE with Set attributes");
                    callback(null);                    
                });
            },
            function(callback) {
                log("Waterfall Three");
                callback(null);
            }
        ], function() {
            log("Waterfall completed!");
        })
    });

Thanks for this, Riley. Steve linked me here thinking that it might offer me a solution to a problem, and I managed to rewrite it to suit my needs. My use-case essentially required treating the tasks array like a stack, so instead of iterating over the array with a closed-over index variable, I'm shift()ing and unshift()ing values.