Firming up our expected order of arguments for the availability method

lib/AvailabilityStrategy.js

@@ -1,29 +1,33 @@
+'use strict';
+
 var AvailabilityStrategy = function() {
 
-    return function() {
-        var sku;
-        var storeIds;
+    return function(sku, storeIds) {
         var options = {
             path: '/',
             qs: {}
         };
 
-        var method;
-
-        for (var i = 0; i < arguments.length; i++) {
-            var arg = arguments[i];
-            if (!isNaN(arg) && typeof arg === 'number') {
-                // SKU
-                sku = arg;
-            } else if (Array.isArray(arg)) {
-                // Store IDs
-                storeIds = arg.join(',');
-            } else if (typeof arg === 'function') {
-                // Result callback
-                options.callback = arg;
+        if (typeof sku !== 'number' && typeof sku !== 'string') {
+            throw 'First parameter of "availability" must be the SKU, and it must be either a number or a string';
+        }
+
+        if (typeof storeIds !== 'number' && Array.isArray(storeIds) === false) {
+            throw 'Second parameter of "availability" must be store id(s), and it must be either a number or array of numbers';
+        } else if (Array.isArray(storeIds)) {
+            storeIds = storeIds.join(',');
+        }
+
+        if (arguments.length === 4) {
+            options.qs = arguments[2];
+            options.callback = arguments[3];
+        } else if (arguments.length === 3) {
+            if (typeof arguments[2] === 'function') {
+                options.callback = arguments[2];
+            } else if (typeof arguments[2] === 'object') {
+                options.qs = arguments[2];
             } else {
-                // Assume any object literal is a set of query params
-                options.qs = arg;
+                throw 'Unrecognized third parameter when calling "availability" method';
             }
         }