flow like the river

BACK_BACK/node_modules/mongoose/lib/helpers/promiseOrCallback.js

@@ -0,0 +1,54 @@
+'use strict';
+
+const immediate = require('./immediate');
+
+const emittedSymbol = Symbol('mongoose:emitted');
+
+module.exports = function promiseOrCallback(callback, fn, ee, Promise) {
+  if (typeof callback === 'function') {
+    try {
+      return fn(function(error) {
+        if (error != null) {
+          if (ee != null && ee.listeners != null && ee.listeners('error').length > 0 && !error[emittedSymbol]) {
+            error[emittedSymbol] = true;
+            ee.emit('error', error);
+          }
+          try {
+            callback(error);
+          } catch (error) {
+            return immediate(() => {
+              throw error;
+            });
+          }
+          return;
+        }
+        callback.apply(this, arguments);
+      });
+    } catch (error) {
+      if (ee != null && ee.listeners != null && ee.listeners('error').length > 0 && !error[emittedSymbol]) {
+        error[emittedSymbol] = true;
+        ee.emit('error', error);
+      }
+
+      return callback(error);
+    }
+  }
+
+  Promise = Promise || global.Promise;
+
+  return new Promise((resolve, reject) => {
+    fn(function(error, res) {
+      if (error != null) {
+        if (ee != null && ee.listeners != null && ee.listeners('error').length > 0 && !error[emittedSymbol]) {
+          error[emittedSymbol] = true;
+          ee.emit('error', error);
+        }
+        return reject(error);
+      }
+      if (arguments.length > 2) {
+        return resolve(Array.prototype.slice.call(arguments, 1));
+      }
+      resolve(res);
+    });
+  });
+};