How to use the cloudinary-core.Util.isString function in cloudinary-core

calc_breakpoint(width, steps) {
    var breakpoints, point;
    breakpoints = (this.state && this.state.breakpoints) || defaultBreakpoints;
    if (Util.isFunction(breakpoints)) {
      return breakpoints(width, steps);
    } else {
      if (Util.isString(breakpoints)) {
        breakpoints = ((function () {
          var j, len, ref, results;
          ref = breakpoints.split(',');
          results = [];
          for (j = 0, len = ref.length; j < len; j++) {
            point = ref[j];
            results.push(parseInt(point));
          }
          return results;
        })()).sort(function (a, b) {
          return a - b;
        });
      }
      return closestAbove(breakpoints, width);
    }
  };