code: function Write(properties) {
                      if (properties)
                          for (var keys = Object.keys(properties), i = 0; i < keys.length; ++i)
                              if (properties[keys[i]] != null)
                                  this[keys[i]] = properties[keys[i]];
                  }
docs: Properties of a Write.
  @memberof google.firestore.v1
  @interface IWrite
  @property {google.firestore.v1.IDocument|null} [update] Write update
  @property {string|null} ["delete"] Write delete
  @property {google.firestore.v1.IDocumentTransform|null} [transform] Write transform
  @property {google.firestore.v1.IDocumentMask|null} [updateMask] Write updateMask
  @property {google.firestore.v1.IPrecondition|null} [currentDocument] Write currentDocument
   
  Constructs a new Write.
  @memberof google.firestore.v1
  @classdesc Represents a Write.
  @implements IWrite
  @constructor
  @param {google.firestore.v1.IWrite=} [properties] Properties to set