concat-map-async.js

var map = require("iterators").map

concatMap.async = concatMapAsync

module.exports = concatMap

function concatMap(xs, fn, thisValue) {
    if (arguments.length === 2) {
        thisValue = this
    }

    var res = [];
    for (var i = 0; i < xs.length; i++) {
        var x = fn.call(thisValue, xs[i], i);
        if (Array.isArray(x)) res.push.apply(res, x);
        else res.push(x);
    }
    return res;
}

function concatMapAsync(xs, fn, thisValue, callback) {
    map(xs, fn, thisValue, function (err, result) {
        if (err) {
            return callback(err)
        }

        var arr = []
        result = arr.concat.apply(arr, result)
        callback(null, result)
    })
}

  if (Array.isArray(x)) res.push.apply(res, x);
  else res.push(x);

res.concat(x);

@michaelficarra

It's substacks code. I'm sure he has a reason for using push.apply instead of concat. Probably because it's destructive when concat is not destructive.