add concatLimit [fixes #1426]

lib/concatLimit.js

@@ -0,0 +1,22 @@
+import doParallelLimit from './internal/doParallelLimit';
+import concat from './internal/concat';
+
+/**
+ * The same as [`concat`]{@link module:Collections.concat} but runs a maximum of `limit` async operations at a time.
+ *
+ * @name concatLimit
+ * @static
+ * @memberOf module:Collections
+ * @method
+ * @see [async.concat]{@link module:Collections.concat}
+ * @category Collection
+ * @param {Array|Iterable|Object} coll - A collection to iterate over.
+ * @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
+ * `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);

lib/index.js

@@ -72,6 +72,7 @@ import autoInject from './autoInject';
 import cargo from './cargo';
 import compose from './compose';
 import concat from './concat';
+import concatLimit from './concatLimit';
 import concatSeries from './concatSeries';
 import constant from './constant';
 import detect from './detect';
@@ -150,6 +151,7 @@ export default {
     cargo: cargo,
     compose: compose,
     concat: concat,
+    concatLimit: concatLimit,
     concatSeries: concatSeries,
     constant: constant,
     detect: detect,
@@ -246,6 +248,7 @@ export {
     cargo as cargo,
     compose as compose,
     concat as concat,
+    concatLimit as concatLimit,
     concatSeries as concatSeries,
     constant as constant,
     detect as detect,

mocha_test/concat.js

@@ -54,4 +54,41 @@ describe('concat', function() {
             done();
         });
     });
+
+    it('concatLimit basics', function(done) {
+        var running = 0;
+        var concurrency = {
+            'foo': 2,
+            'bar': 2,
+            'baz': 1
+        };
+
+        async.concatLimit(['foo', 'bar', 'baz'], 2, function(val, next) {
+            running++;
+            async.setImmediate(function() {
+                expect(running).to.equal(concurrency[val]);
+                running--;
+                next(null, [val, val]);
+            });
+        }, function(err, result) {
+            expect(running).to.equal(0);
+            expect(err).to.eql(null);
+            expect(result).to.eql(['foo', 'foo', 'bar', 'bar', 'baz', 'baz']);
+            done();
+        });
+    });
+
+    it('concatLimit error', function(done) {
+        var arr = ['foo', 'bar', 'baz'];
+        async.concatLimit(arr, 2, function(val, next) {
+            if (val === 'bar') {
+                return next(new Error('fail'));
+            }
+            next(null, [val, val]);
+        }, function(err, result) {
+            expect(err).to.not.eql(null);
+            expect(result).to.eql(['foo', 'foo']);
+            done();
+        });
+    });
 });