fix(define-operation): ensure optional parameters are considered

In one case (`checkPassword`) we support an optional parameter, so
`defineOperation` needs to take this into account, and modify the
current parameter index during validation.

fixes NODE-1770

lib/util.js

@@ -10,12 +10,16 @@ function validateParameter(parameter, spec) {
   }
 
   if (spec.type && typeof parameter !== spec.type) {
+    if (spec.required === false) return false;
+
     throw new TypeError(
       `Invalid type for parameter \`${spec.name}\`, expected \`${
         spec.type
       }\` but found \`${typeof parameter}\``
     );
   }
+
+  return true;
 }
 
 /**
@@ -30,11 +34,11 @@ function defineOperation(fn, paramDefs) {
   return function() {
     const args = Array.prototype.slice.call(arguments);
     const params = [];
-    for (let i = 0; i < paramDefs.length; ++i) {
+    for (let i = 0, argIdx = 0; i < paramDefs.length; ++i, ++argIdx) {
       const def = paramDefs[i];
-      let arg = args[i];
+      let arg = args[argIdx];
 
-      if (def.default && arg == null) arg = def.default;
+      if (def.hasOwnProperty('default') && arg == null) arg = def.default;
       if (def.type === 'object' && def.default != null) {
         arg = Object.assign({}, def.default, arg);
       }
@@ -44,11 +48,14 @@ function defineOperation(fn, paramDefs) {
         arg = {};
       }
 
-      validateParameter(arg, paramDefs[i]);
-      params.push(arg);
+      if (validateParameter(arg, paramDefs[i])) {
+        params.push(arg);
+      } else {
+        argIdx--;
+      }
     }
 
-    const callback = params.pop();
+    const callback = arguments[arguments.length - 1];
     if (typeof callback !== 'function') {
       return new Promise((resolve, reject) => {
         params.push((err, response) => {
@@ -60,7 +67,6 @@ function defineOperation(fn, paramDefs) {
       });
     }
 
-    params.push(callback);
     fn.apply(this, params);
   };
 }