Get or set a value on a layer in Sketch

getSetValueOnLayer.js

var value = nil,
	key = "kModelPropertiesKey",
	layer = context.selection[0],
	plugin = "com.animaapp.stc-sketch-plugin";

// Get the value
context.command.valueForKey_onLayer_forPluginIdentifier(key,layer,plugin);

// Set the value
context.command.setValue_forKey_onLayer_forPluginIdentifier(value,key,layer,plugin);