transaction now returns a promise

src/database.js

@@ -102,16 +102,20 @@ function query() {
     });
 }
 
-function transaction(queries, callback) {
+function transaction(queries) {
     assert(Array.isArray(queries));
-    assert.strictEqual(typeof callback, 'function');
 
-    callback = once(callback);
+    const args = Array.prototype.slice.call(arguments);
+    const callback = typeof args[args.length - 1] === 'function' ? once(args.pop()) : null;
 
+    return new Promise((resolve, reject) => {
         gConnectionPool.getConnection(function (error, connection) {
-        if (error) return callback(error);
+            if (error) return callback ? callback(error) : reject(new BoxError(BoxError.DATABASE_ERROR, error, { code: error.code, sqlMessage: error.sqlMessage }));
 
-        const releaseConnection = (error) => { connection.release(); callback(error); };
+            const releaseConnection = (error) => {
+                connection.release();
+                callback ? callback(error) : reject(new BoxError(BoxError.DATABASE_ERROR, error, { code: error.code, sqlMessage: error.sqlMessage }));
+            };
 
             connection.beginTransaction(function (error) {
                 if (error) return releaseConnection(error);
@@ -126,7 +130,8 @@ function transaction(queries, callback) {
 
                         connection.release();
 
-                    callback(null, results);
+                        callback ? callback(null, results) : resolve(results);
+                    });
                 });
             });
         });