Merge pull request #15566 from Automattic/vkarpov15/gh-15551

fix(model): remove unnecessary conversion of undefined -> null in findById

Author: vkarpov15

lib/model.js

@@ -2092,21 +2092,14 @@ Model.find = function find(conditions, projection, options) {
 };
 
 /**
- * Finds a single document by its _id field. `findById(id)` is almost*
- * equivalent to `findOne({ _id: id })`. If you want to query by a document's
- * `_id`, use `findById()` instead of `findOne()`.
+ * Finds a single document by its _id field. `findById(id)` is equivalent to `findOne({ _id: id })`.
  *
  * The `id` is cast based on the Schema before sending the command.
  *
  * This function triggers the following middleware.
  *
  * - `findOne()`
  *
- * \* Except for how it treats `undefined`. If you use `findOne()`, you'll see
- * that `findOne(undefined)` and `findOne({ _id: undefined })` are equivalent
- * to `findOne({})` and return arbitrary documents. However, mongoose
- * translates `findById(undefined)` into `findOne({ _id: null })`.
- *
  * #### Example:
  *
  *     // Find the adventure with the given `id`, or `null` if not found
@@ -2131,10 +2124,6 @@ Model.findById = function findById(id, projection, options) {
     throw new MongooseError('Model.findById() no longer accepts a callback');
   }
 
-  if (typeof id === 'undefined') {
-    id = null;
-  }
-
   return this.findOne({ _id: id }, projection, options);
 };