A task in your function_array can only add a new task to the array provided it is NOT the last task in the array.
In your case, your function_array contained only 1 task. That task itself cannot add additional tasks since it's the last task.
The solution is to have 2 tasks in the array. A startTask to bootstrap the process, and a finalTask that is more of a dummy task. In that case,
function_array = [startTask, finalTask];
Then startTask would add taskA, taskB will add task C and eventually
function_array = [startTask, taskA, taskB, taskC, finalTask];
The sample code below that illustrates the concepts.
var async = require('async');
var max = 6;
var nodeTask = function(taskId, value, callback){
var r = Math.floor(Math.random() * 20) + 1;
console.log("From Node Task %d: %d", taskId, r);
// add an edge task
if (taskId < max) {
function_array.splice(function_array.length-1, 0, edgeTask);
}
callback(null, taskId + 1, value + r);
};
var edgeTask = function(taskId, value, callback){
var r = Math.floor(Math.random() * 20) + 1;
console.log("From Edge Task %d: %d", taskId, r);
// add a node task
if (taskId < max) {
function_array.splice(function_array.length-1, 0, nodeTask);
}
callback(null, taskId + 1, value + r);
};
var startTask = function(callback) {
function_array.splice(function_array.length-1, 0, nodeTask);
callback(null, 1, 0);
};
var finalTask = function(taskId, value, callback) {
callback(null, value);
};
var function_array = [startTask, finalTask];
async.waterfall(function_array, function (err, result) {
console.log("Sum is ", result);
});