waterfall = function(tasks, callback) {
    callback = once(callback || noop);
    if (!isArray(tasks)) return callback(new Error('First argument to waterfall must be an array of functions'));
    if (!tasks.length) return callback();
    var taskIndex = 0;

    function nextTask(args) {
        var task = wrapAsync(tasks[taskIndex++]);
        args.push(onlyOnce(next));
        task.apply(null, args);
    }

    function next(err/*, ...args*/) {
        if (err || taskIndex === tasks.length) {
            return callback.apply(null, arguments);
        }
        nextTask(slice(arguments, 1));
    }

    nextTask([]);
}
Runs the `tasks` array of functions in series, each passing their results to
the next in the array. However, if any of the `tasks` pass an error to their
own callback, the next function is not executed, and the main `callback` is
immediately called with the error.

@name waterfall
@static
@memberOf module:ControlFlow
@method
@category Control Flow
@param {Array} tasks - An array of [async functions]{@link AsyncFunction}
to run.
Each function should complete with any number of `result` values.
The `result` values will be passed as arguments, in order, to the next task.
@param {Function} [callback] - An optional callback to run once all the
functions have completed. This will be passed the results of the last task's
callback. Invoked with (err, [results]).
@returns undefined
@example

async.waterfall([
    function(callback) {
        callback(null, 'one', 'two');
    },
    function(arg1, arg2, callback) {
        // arg1 now equals 'one' and arg2 now equals 'two'
        callback(null, 'three');
    },
    function(arg1, callback) {
        // arg1 now equals 'three'
        callback(null, 'done');
    }
], function (err, result) {
    // result now equals 'done'
});

// Or, with named functions:
async.waterfall([
    myFirstFunction,
    mySecondFunction,
    myLastFunction,
], function (err, result) {
    // result now equals 'done'
});
function myFirstFunction(callback) {
    callback(null, 'one', 'two');
}
function mySecondFunction(arg1, arg2, callback) {
    // arg1 now equals 'one' and arg2 now equals 'two'
    callback(null, 'three');
}
function myLastFunction(arg1, callback) {
    // arg1 now equals 'three'
    callback(null, 'done');
}