preserve order, make variadic and handle falsy values in concat [fixes #1437] (#1436)

* preserve order in concat

* remove comment

* fix concatLimit linting

* handle variadic and falsy results in concatLimit [fixes #1437]

* fix tests (typos)

* PR fixes

1 files changed, 26 insertions, 4 deletions

diff --git a/lib/concatLimit.js b/lib/concatLimit.js
index a86fd74..4df03ea 100644
--- a/lib/concatLimit.js
+++ b/lib/concatLimit.js
@@ -1,5 +1,9 @@
-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';
+
+var _concat = Array.prototype.concat;
 
 /**
  * The same as [`concat`]{@link module:Collections.concat} but runs a maximum of `limit` async operations at a time.
@@ -14,9 +18,27 @@ 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 = [];
+        for (var i = 0; i < mapResults.length; i++) {
+            if (mapResults[i]) {
+                result = _concat.apply(result, mapResults[i]);
+            }
+        }
+
+        return callback(err, result);
+    });
+}