@atlaskit/adf-schema/dist/esm/schema/nodes/copy-private-attributes.js

Shared package that contains the ADF-schema (json) and ProseMirror node/mark specs

export var copyPrivateAttributes = function copyPrivateAttributes(from, to, attributes, map) {
  var attrs = attributes || {};
  Object.keys(attrs).forEach(function (key) {
    if (key[0] === '_' && key[1] === '_' && from[key]) {
      to[map ? map(key) : key] = from[key];
    }
  });
};