Add util.negate

This function takes an (asynchronous) function that is expected to
return a boolean and returns a new function which returns the inverse of
the original function.

lib/util.js

@@ -198,4 +198,16 @@ util.isEmail = function isEmail(email) {
   return reEmail.test(email);
 };
 
+util.negate = function negate(fn) {
+  return function () {
+    const args = [].slice.call(arguments);
+    const cb = args.pop();
+    args.push(function (err, result) {
+      if (err) return cb(err);
+      return cb(null, !result);
+    });
+    fn.apply(null, args);
+  };
+};
+
 module.exports = util;

tests/util.test.js

@@ -98,3 +98,13 @@ test('util.whitelist', function (t) {
   t.same(list.exempt('AaaaaaaD'), true);
   t.end();
 });
+
+test('util.negate', function (t) {
+  function asyncTrue(cb) {
+    cb(null, true);
+  };
+  util.negate(asyncTrue)(function (err, result) {
+    t.same(result, false);
+    t.end();
+  });
+});