The generated function of seq requires last argument to be function?

[mocheng]

The sample code here https://github.com/caolan/async#seq looks that generated function of seq can work with only one argument.

However, below code breaks

var async = require('async');

var init_arg = 3;

async.seq(
    function (num, next) {
        console.log(num);
        console.log(next);
        next(null, num + 1);
    },
    function (num, next) {
        console.log(num);
        console.log(next);

        next(null, num + 3);
    }
)(init_arg);

The output is like:

/Users/chengmo/stash/beeper/test.js:9
next(null, num + 1);
^
TypeError: undefined is not a function
at /Users/chengmo/stash/beeper/test.js:9:3
at /Users/chengmo/stash/beeper/node_modules/async/lib/async.js:1058:20
at /Users/chengmo/stash/beeper/node_modules/async/lib/async.js:272:13
at iterate (/Users/chengmo/stash/beeper/node_modules/async/lib/async.js:149:13)
at Object.async.eachSeries (/Users/chengmo/stash/beeper/node_modules/async/lib/async.js:165:9)
at Object.async.reduce (/Users/chengmo/stash/beeper/node_modules/async/lib/async.js:271:15)
at /Users/chengmo/stash/beeper/node_modules/async/lib/async.js:1057:19
at Object. (/Users/chengmo/stash/beeper/test.js:22:2)
at Module._compile (module.js:456:26)
at Object.Module._extensions..js (module.js:474:10)

[aearly]

Specify a callback function:

var async = require('async');

var init_arg = 3;

async.seq(
    function (num, next) {
        console.log(num);
        console.log(next);
        next(null, num + 1);
    },
    function (num, next) {
        console.log(num);
        console.log(next);

        next(null, num + 3);
    }
)(init_arg, function (err, result) {
    //...
});

The documentation is a bit poor since it doesn't have one in its example, but the callback is needed in order to work.

[mocheng]

In some cases, I suppose in most cases, no callback is wanted for seq. No callback means no-op callback would be better.

[aearly]

I agree -- it would be pretty simple to add a noop callback if none exists. PR's are welcome, @caolan will get to them, eventually....