code: function eval$if(params) {
    var script = params.$if
                ? params.$then
                : typeof params.$else == 'undefined' ? null : params.$else;
    return isScript(script)
            ? new this.js.Script(script)
            : script;
  }
docs: conditional evaluation
  @this   Evaluation
  @param  {Object} params instruction keywords ($if, $then, $else)
  @return {Script|Any} script to be evaluated