docs: function(str, obj) {
        var isStringOrNumber = function(a) {
          var t = typeof a;
          return t === 'string' || t === 'number';
        };
  
        return str.replace(/\${([^{}]*)}/g,
          function (a, b) {
            var value = obj[b];
            return isStringOrNumber(value) ? value : a;
          }
        );
      }