Fix: Verify arguments to series/parallel functions (fixes #4)

lib/createParallel.js

@@ -2,11 +2,12 @@
 
 var _ = require('lodash');
 var async = require('async');
+var verifyArguments = require('./verifyArguments');
 
 function createParallel(mapFn){
 
   function buildParallel(){
-    var args = _.flatten(arguments);
+    var args = verifyArguments(_.flatten(arguments));
 
     function parallel(done){
       async.map(args, mapFn, done);

lib/createSeries.js

@@ -2,11 +2,12 @@
 
 var _ = require('lodash');
 var async = require('async');
+var verifyArguments = require('./verifyArguments');
 
 function createSeries(mapFn){
 
   function buildSeries(){
-    var args = _.flatten(arguments);
+    var args = verifyArguments(_.flatten(arguments));
 
     function series(done){
       async.mapSeries(args, mapFn, done);

lib/verifyArguments.js

@@ -0,0 +1,13 @@
+var assert = require('assert');
+
+module.exports = function(args){
+
+  assert(args.length > 0, 'A set of functions to combine is mandatory');
+
+  args.forEach(function(arg, argIdx){
+    assert.equal(typeof arg, 'function',
+        'Only functions can be combined, got ' + typeof arg + ' for argument ' + argIdx);
+  });
+
+  return args;
+};

test/verifyArguments.js

@@ -0,0 +1,32 @@
+'use strict';
+
+var test = require('tap').test;
+
+var verifyArguments = require('../lib/verifyArguments');
+
+function validArg() {
+}
+
+test('should act as pass-through for a valid set of arguments', function(t){
+  var args = [validArg, validArg];
+  t.deepEqual(verifyArguments(args), args);
+  t.end();
+});
+
+test('should throw descriptive error message on invalid argument', function(t){
+  t.throws(function(){
+    verifyArguments([validArg, 'invalid', validArg]);
+  }, {
+    name: 'AssertionError', message: 'Only functions can be combined, got string for argument 1'
+  }, 'should throw AssertionError');
+  t.end();
+});
+
+test('should throw descriptive error message on when no arguments provided', function(t){
+  t.throws(function(){
+    verifyArguments([]);
+  }, {
+    name: 'AssertionError', message: 'A set of functions to combine is mandatory'
+  }, 'should throw AssertionError');
+  t.end();
+});