code: function parseNumberArray(array) {
      var widths = array.map(parseNumber);
  
      return function(z) {
          var result = [];
          for (var i = 0; i < widths.length; i++) {
              result.push(typeof widths[i] === 'function' ? widths[i](z) : widths[i]);
          }
          return result;
      };
  }
docs: from mapbox-gl-js/js/style/style_declaration.js