preserve order, make variadic and handle falsy values in concat

lib/concat.js

@@ -1,5 +1,5 @@
-import concat from './internal/concat';
-import doParallel from './internal/doParallel';
+import doLimit from './internal/doLimit';
+import concatLimit from './concatLimit';
 
 /**
  * Applies `iteratee` to each item in `coll`, concatenating the results. Returns
@@ -26,4 +26,4 @@ import doParallel from './internal/doParallel';
  *     // files is now a list of filenames that exist in the 3 directories
  * });
  */
-export default doParallel(concat);
+export default doLimit(concatLimit, Infinity);

lib/concatLimit.js

@@ -1,5 +1,7 @@
-import doParallelLimit from './internal/doParallelLimit';
-import concat from './internal/concat';
+import noop from 'lodash/noop';
+import wrapAsync from './internal/wrapAsync';
+import slice from './internal/slice';
+import mapLimit from './mapLimit';
 
 /**
  * The same as [`concat`]{@link module:Collections.concat} but runs a maximum of `limit` async operations at a time.
@@ -14,9 +16,28 @@ import concat from './internal/concat';
  * @param {number} limit - The maximum number of async operations at a time.
  * @param {AsyncFunction} iteratee - A function to apply to each item in `coll`,
  * which should use an array as its result. Invoked with (item, callback).
- * @param {Function} [callback(err)] - A callback which is called after all the
+ * @param {Function} [callback] - A callback which is called after all the
  * `iteratee` functions have finished, or an error occurs. Results is an array
  * containing the concatenated results of the `iteratee` function. Invoked with
  * (err, results).
  */
-export default doParallelLimit(concat);
+export default function(coll, limit, iteratee, callback) {
+    callback = callback || noop;
+    var _iteratee = wrapAsync(iteratee);
+    mapLimit(coll, limit, function(val, callback) {
+        _iteratee(val, function(err /*, ...args*/) {
+            if (err) return callback(err);
+            return callback(null, slice(arguments, 1));
+        });
+    }, function(err, mapResults) {
+        var result = [];
+        var _concat = Array.prototype.concat;
+        for (var i = 0; i < mapResults.length; i++) {
+            if (mapResults[i]) {
+                result = _concat.apply(result, mapResults[i]);
+            }
+        }
+
+        return callback(err, result);
+    });
+}

lib/concatSeries.js

@@ -1,5 +1,5 @@
-import concat from './internal/concat';
-import doSeries from './internal/doSeries';
+import doLimit from './internal/doLimit';
+import concatLimit from './concatLimit';
 
 /**
  * The same as [`concat`]{@link module:Collections.concat} but runs only a single async operation at a time.
@@ -19,4 +19,4 @@ import doSeries from './internal/doSeries';
  * containing the concatenated results of the `iteratee` function. Invoked with
  * (err, results).
  */
-export default doSeries(concat);
+export default doLimit(concatLimit, 1);