test: add test coverage for parallelLimit()

lib/helpers/parallelLimit.js

@@ -11,11 +11,15 @@ function parallelLimit(fns, limit, callback) {
   let numFinished = 0;
   let error = null;
 
+  if (limit <= 0) {
+    throw new Error('Limit must be positive');
+  }
+
   if (fns.length === 0) {
     return callback(null, []);
   }
 
-  for (let i = 0; i < fns.length && i <= limit; ++i) {
+  for (let i = 0; i < fns.length && i < limit; ++i) {
     _start();
   }
 

test/parallelLimit.test.js

@@ -0,0 +1,43 @@
+'use strict';
+
+const assert = require('assert');
+const parallelLimit = require('../lib/helpers/parallelLimit');
+
+describe('parallelLimit', function() {
+  it('works with zero functions', function(done) {
+    parallelLimit([], 1, (err, res) => {
+      assert.ifError(err);
+      assert.deepEqual(res, []);
+      done();
+    });
+  });
+
+  it('executes functions in parallel', function(done) {
+    let called = 0;
+    const fns = [
+      cb => {
+        setTimeout(() => {
+          ++called;
+          setTimeout(cb, 0);
+        }, 100);
+      },
+      cb => {
+        setTimeout(() => {
+          ++called;
+          setTimeout(cb, 0);
+        }, 100);
+      },
+      cb => {
+        assert.equal(called, 2);
+        ++called;
+        setTimeout(cb, 100);
+      }
+    ];
+
+    parallelLimit(fns, 2, (err) => {
+      assert.ifError(err);
+      assert.equal(called, 3);
+      done();
+    });
+  });
+});