Get the corner radius of an existing vector shape layer?

Coming back to my own question after a couple years:
Turns out, each layer in PS has a 'json' property you can get from a bit of ActionManager code.
(I learned how to write this from jazz-y. Thanks!):

var r = new ActionReference();
r.putProperty(charIDToTypeID('Prpr'), stringIDToTypeID('json'));
r.putEnumerated(stringIDToTypeID("layer"), stringIDToTypeID("ordinal"), stringIDToTypeID("targetEnum"));
var thisJson = executeActionGet(r).getString(stringIDToTypeID('json'));
alert(thisJson);

The JSON string returned has a wealth of info about a layer (depending on the type of layer). In the case of a rounded rectangle it returns this lovely string:

{
"version": "1.6.1",
"timeStamp": 1609818097.180,
"count": 138,
"id": 897,
"file": "\/Users\/steve\/Dropbox\/_ToolsLocal\/_ScriptsLocal\/Photoshop\/Shapes\/ColorStyleProcessing\/textSample.psd",
"bounds": {
"top": 0,
"left": 0,
"bottom": 768,
"right": 1024
},
"selection": [1],
"resolution": 72,
"globalLight": {
"angle": 90,
"altitude": 30
},
"generatorSettings": false,
"profile": "Display P3",
"mode": "RGBColor",
"depth": 8,
"layers": [{
"id": 122,
"index": 1,
"type": "shapeLayer",
"name": "Rounded Rectangle 1",
"bounds": {
"top": 161,
"left": 467,
"bottom": 348,
"right": 594
},
"visible": true,
"clipped": false,
"fill": {
"color": {
"red": 203.626,
"green": 38.0039,
"blue": 213.988
},
"class": "solidColorLayer"
},
"strokeStyle": {
"strokeStyleVersion": 2,
"strokeEnabled": false,
"fillEnabled": true,
"strokeStyleLineWidth": 2,
"strokeStyleLineDashOffset": {
"value": 0,
"units": "pointsUnit"
},
"strokeStyleMiterLimit": 100,
"strokeStyleLineCapType": "strokeStyleButtCap",
"strokeStyleLineJoinType": "strokeStyleMiterJoin",
"strokeStyleLineAlignment": "strokeStyleAlignCenter",
"strokeStyleScaleLock": false,
"strokeStyleStrokeAdjust": false,
"strokeStyleBlendMode": "normal",
"strokeStyleOpacity": {
"value": 100,
"units": "percentUnit"
},
"strokeStyleContent": {
"color": {
"red": 247,
"green": 225.689,
"blue": 225.689
}
},
"strokeStyleResolution": 72
},
"generatorSettings": false,
"path": {
"pathComponents": [{
"origin": {
"radii": [2, 2, 40, 40],
"type": "roundedRect",
"bounds": {
"top": 161,
"left": 467,
"bottom": 348,
"right": 594
}
}
}],
"bounds": {
"top": 161,
"left": 467,
"bottom": 348,
"right": 594
},
"defaultFill": false
}
}]
}

You'll notice that under 'pathComponents', there's something called 'radii'. (Which as far as I know, is only a property used for rounded rectangles). The following numbers in square brackets are your corner radii:

"radii": [2, 2, 40, 40],

Parsing JSON in ExtendScript (basically a 20-year old version of Javascript) isn't a good time, but if you're just trying to get one specific thing it's not so bad. You can use splitting:

// split the string in halves at 'radii' and grab the second half:
var myRads = thisJson.split("\"radii\":\[")[1];

// then split again, grabbing the first segment before the ']':
myRads = myRads.split("\]")[0]; //returns '2, 2, 40, 40';

// one more split gives you an array of the 4 numbers:
myRads = myRads.split(',');

// See results:
var str = "topRight: "+myRads[0]+"\n"+
          "bottomRight: "+myRads[1]+"\n"+
          "bottomLeft: "+myRads[2]+"\n"+
          "topLeft: "+myRads[3];
alert(str);

/*  Results:
topRight: 2
bottomRight: 2
bottomLeft: 40
topLeft: 40
*/