gistfile1.js

properties: {
  title: {
    type: 'string',
    
    conditional: {
      optional: {
        value: true,
        when: function () {         // The 'optional' attribute only takes effect if this function returns true
          return !this.published;   // So the title is only optional if the article hasn't been published.
        }
      },
      maxLength: { value: 140,  when: function () {} }
  }
}