Simple example Sketch plugin script to export layers into json data. This was an experiment. It's not pretty.

script.cocoascript

// Sketch export layers to json script
// This was hacked together for experimentation purposes, the code is ugly.
// logs:
// tail -f /var/log/system.log | grep Sketch

//I only later realized that there are methods to get the parent or children
//layers... oops. Seriously, I just threw this together.
function makeChildrenArray(layerGroup, dict, allLayers) {
  // log("called makeChildrenArray for " + layerGroup)
  // result = new Array()
  for (var i = 0; i < layerGroup.layers().count(); i++) {

    someLayer = layerGroup.layers().objectAtIndex(i)

    if ([someLayer isKindOfClass:[MSShapePathLayer class]]) {
      continue;
    }

    dict[someLayer] = layerGroup
    allLayers.push(someLayer)
    if ([someLayer isKindOfClass:[MSLayerGroup class]]) {
      sub = makeChildrenArray(someLayer, dict, allLayers)
    }
  }
}


var onRun = function(context) {

  var documentName = context.document.displayName();
  log('The current document is named: ' + documentName);

  var selectedLayers = context.selection;
  var selectedCount = selectedLayers.count();

  if (selectedCount == 0) {
    log('No layers are selected.');
  } else {
    // var layer = selectedLayers[0];
    log('Selected layer: ' + selectedLayers[0]);
    var allLayersParents = {}
    var allLayers = []

    makeChildrenArray(selectedLayers[0], allLayersParents, allLayers);

    var layerInfo = {}
    layerInfo["layers"] = []

    //add root layer
    layerInfo["root"] = {}
    layerInfo["root"]["descriptionKey"] = String(selectedLayers[0])
    layerInfo["root"]["name"] = "rootView"

    //there must be a prettier way to extract these variables in javascript.
    //feel free to enlighten me.
    for (i = 0; i < allLayers.length; i++) {
      var layer = allLayers[i];

      var info = {}

      var frame = layer.rect()
      info["frame"] = {}
      info["frame"]["x"] = String(frame.origin.x.toString());
      info["frame"]["y"] = String(frame.origin.y);
      info["frame"]["width"] = String(frame.size.width);
      info["frame"]["height"] = String(frame.size.height);

      info["type"] = String([layer class])
      info["name"] = String([layer name])
      info["isVisible"] = String([layer isVisible])
      info["descriptionKey"] = String([layer description])
      info["descriptionKeyParent"] = String(allLayersParents[layer])

      if ([layer isKindOfClass:[MSTextLayer class]]) {
        info["text"] = {}
        info["text"]["stringValue"] = String([layer stringValue])
        info["text"]["textAlignment"] = String([layer textAlignment])
        info["text"]["textColor"] = String([layer textColor])
        info["text"]["fontSize"] = String([layer fontSize])
        info["text"]["fontPostscriptName"] = String([layer fontPostscriptName])

      }

      if ([layer isKindOfClass:[MSStyleBorder class]]) {
        info["border"] = {}
        info["border"]["position"] = String([layer position])
        info["border"]["thickness"] = String([layer thickness])
        info["border"]["fillType"] = String([layer fillType])
        info["border"]["gradient"] = String([layer gradient])
        info["border"]["isEnabled"] = String([layer isEnabled])
      }

      if ([layer isKindOfClass:[MSStyleFill class]]) {
        info["fill"] = {}
        info["fill"]["fillType"] = String([layer fillType])
        info["fill"]["colorGeneric"] = String([layer colorGeneric])
        info["fill"]["image"] = String([layer image])
        info["fill"]["noiseIntensity"] = String([layer noiseIntensity])
        info["fill"]["isEnabled"] = String([layer isEnabled])
      }

      if ([layer isKindOfClass:[MSStyleInnerShadow class]]) {
        info["innerShadow"] = {}
        info["innerShadow"]["offsetX"] = String([layer offsetX])
        info["innerShadow"]["offsetY"] = String([layer offsetY])
        info["innerShadow"]["blurRadius"] = String([layer blurRadius])
        info["innerShadow"]["spread"] = String([layer spread])
        info["innerShadow"]["color"] = String([layer color])
        info["innerShadow"]["isEnabled"] = String([layer isEnabled])
      }

      if ([layer isKindOfClass:[MSStyleInnerShadow class]]) {
        info["shadow"] = {}
        info["shadow"]["offsetX"] = String([layer offsetX])
        info["shadow"]["offsetY"] = String([layer offsetY])
        info["shadow"]["blurRadius"] = String([layer blurRadius])
        info["shadow"]["spread"] = String([layer spread])
        info["shadow"]["color"] = String([layer color])
        info["shadow"]["isEnabled"] = String([layer isEnabled])
      }

      if (layer.style) {
        if (layer.style().fills().array().count() > 0) {
          // log("has fill: " + layer.style().fills().array())
          fill = layer.style().fills().array()[0]
          log(fill)
          info["style"] = {}
          info["style"]["fill"] = {}
          info["style"]["fill"]["fillType"] = String([fill fillType])
          info["style"]["fill"]["colorGeneric"] = String([fill colorGeneric])
          info["style"]["fill"]["isEnabled"] = String([fill isEnabled])
        }

        if (layer.style().borders().array().count() > 0) {
          // log("has fill: " + layer.style().fills().array())
          border = layer.style().borders().array()[0]
          log(border)
          if (!("style" in info)) {
            info["style"] = {}
            log("added style")
          }
          if (!("border" in info["style"])) {
            info["style"]["border"] = {}
            log("added style border")
          }
          info["style"]["border"]["fillType"] = String([border fillType])
          info["style"]["border"]["position"] = String([border position])
          info["style"]["border"]["thickness"] = String([border thickness])
          info["style"]["border"]["colorGeneric"] = String([border colorGeneric])
          info["style"]["border"]["isEnabled"] = String([border isEnabled])
        }
      }

      layerInfo["layers"].push(info)
    }

    var someString = [NSString stringWithFormat:"%@", JSON.stringify(layerInfo, null, 2)],
    filePath = "/Users/"+ NSUserName() +"/Desktop/layerInfo.json";
    [someString writeToFile:filePath atomically:true encoding:NSUTF8StringEncoding error:nil];
  };
};