reduce callback isn't really optional

[asquared]

What version of async are you using?
1.5.2 (but the offending code seems to still exist in master)

Which environment did the issue occur in (Node version/browser version)
node v0.10.29

What did you do? Please include a minimal reproducable case illustrating issue.
The documentation says the callback is optional, so I should be able to do:

const async = require('async');

var array = [1, 2, 3];
function reducer(memo, item, callback) {
        memo += item;
        console.log("sum is %d", memo);
        setImmediate(callback, null, memo);
}

async.reduce(array, 0, reducer);

What did you expect to happen?

sum is 1
sum is 3
sum is 6

What was the actual result?

sum is 1
sum is 3
sum is 6

.../node_modules/async/lib/async.js:380
            callback(err, memo);
            ^
TypeError: undefined is not a function
    at .../node_modules/async/lib/async.js:380:13
    at .../node_modules/async/lib/async.js:52:16
    at .../node_modules/async/lib/async.js:269:32
    at .../node_modules/async/lib/async.js:44:16
    at Object.<anonymous> (.../node_modules/async/lib/async.js:377:17)
    at Object.immediate._onImmediate (timers.js:354:16)
    at processImmediate [as _immediateCallback] (timers.js:336:15)

[megawac]

Yeah, actually looking into this a lot of methods are documented to have optional callbacks when they are clearly required

These are the ones I've identified are incorrectly documented:

• filter*
• map*
• reduce*
• sortBy
• transform

[megawac]

Should these be optional or not @aearly

[asquared]

In my case it's helpful if they're optional. At each step of the reduce I can delete a file until a certain amount of data has been removed. The total bytes remaining to be deleted are passed along in the memo. I don't care about the final result - just the side effect of files getting deleted - so I'm using an empty callback.

[aearly]

Yeah, they should all be optional. I do think it's odd that you would use map or filter and then ignore the result, but we should make the callback optional for consistency with other methods.

[megawac]

I think sortBy should be the only one required -- any objection @aearly?

[aearly]

Yeah, there'd be no reason to use sortBy and ignore the result.