[JavaScript (with promises)] Bug following $ref in object properties

Description

Generated type file ID contains the following:

  /**
   * Constructs a <code>ID</code> from a plain JavaScript object, optionally creating a new instance.
   * Copies all relevant properties from <code>data</code> to <code>obj</code> if supplied or a new instance if not.
   * @param {Object} data The plain JavaScript object bearing properties of interest.
   * @param {module:model/ID} obj Optional instance to populate.
   * @return {module:model/ID} The populated <code>ID</code> instance.
   */
  exports.constructFromObject = function(data, obj) {
    if (data) {
      obj = obj || new exports();

    }
    return obj;
  }

data is an integer input. obj is null. Hence, the result is exports() instead of simply data.

openapi-generator version

openapi-generator-cli-3.3.3-20181113.090410-41.jar

OpenAPI declaration file content or url

definitions:

  # ===============================================================================
  # Fragments
  # ===============================================================================

  ID:
    description: An entity identifer
    type: integer
    format: int64
    readOnly: true

  # ===========================================================================
  # Users
  # ===========================================================================

  User:
    type: object
    required:
      - emailAddress
    properties:
      id:
        $ref: '#/definitions/ID'
      emailAddress:
        type: string
        format: email
        minLength: 6
        maxLength: 254

Command line used for generation

java -jar openapi-generator-cli-3.3.3-20181113.090410-41.jar generate -i ../source.yaml -l javascript --additional-properties usePromises=true -o ./javascript/

Steps to reproduce

Use the above definitions in any operation. You'll get a result object with an exports() objects instead of an ID integer.

Related issues/PRs

This is the same issue in swagger-codegen:
https://github.com/swagger-api/swagger-codegen/issues/4973

Suggest a fix/enhancement

It tempts me to say:

  /**
   * Constructs a <code>ID</code> from a plain JavaScript object, optionally creating a new instance.
   * Copies all relevant properties from <code>data</code> to <code>obj</code> if supplied or a new instance if not.
   * @param {Object} data The plain JavaScript object bearing properties of interest.
   * @param {module:model/ID} obj Optional instance to populate.
   * @return {module:model/ID} The populated <code>ID</code> instance.
   */
  exports.constructFromObject = function(data, obj) {
    if (data) {
      obj = **data** || new exports();

    }
    return obj;
  }

but I actually think the issue is that the generator doesn't follow the $ref properly. Not certain

Reproduce the issue with the supplied OpenAPI definitions and Java generation command, then inspect the generated JavaScript ID model's constructFromObject function and the generator path that produced it. Done means a scalar integer reached through the ID $ref remains an integer instead of becoming an empty ID object, with a regression check covering the example.