@atlaskit/adf-schema/dist/es2019/schema/marks/to-json.js

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

const OPTIONAL_ATTRS = ['title', 'id', 'collection', 'occurrenceKey', '__confluenceMetadata'];
export const toJSON = mark => ({
  type: mark.type.name,
  attrs: Object.keys(mark.attrs).reduce((attrs, key) => {
    if (OPTIONAL_ATTRS.indexOf(key) === -1 || mark.attrs[key] !== null) {
      attrs[key] = mark.attrs[key];
    }
    return attrs;
  }, {})
});