the __stack__ property becomes visible after accessing Error.stack -

var err = new Error("the-message");
print('err.json1: '+JSON.stringify(err));
print('err: '+err);
print('err.message: '+err.message);
print('err.stack: '+err.stack);
print('err.json2: '+JSON.stringify(err));

this prints -

err.json1: {}
err: Error: the-message
err.message: the-message
err.stack: Error: the-message
	at <program> (error.js:1)
err.json2: {"__stack__":"Error: the-message\n\tat <program> (error.js:1)"}

node/v8 prints -

err.json1: {}
err: Error: the-message
err.message: the-message
err.stack: Error: the-message
    ...
    at node.js:902:3
err.json2: {}