The Error class in node.js

Question

Obviously in console core modules(https://nodejs.org/dist/latest-v4.x/docs/api/console.html) of node documentation exist below code:

console.error(new Error('Whoops, something bad happened'));
  // Prints: [Error: Whoops, something bad happened], to stderr

but,when I run test.js which code show below

var err = new Error('a');
console.error(err);

the terminal print the message like:

Error: a at Object. (/Users/suoyong/Express/连接数据库/error.js:1:73) at Module._compile (module.js:556:32) at Object.Module._extensions..js (module.js:565:10) at Module.load (module.js:473:32) at tryModuleLoad (module.js:432:12) at Function.Module._load (module.js:424:3) at Module.runMain (module.js:590:10) at run (bootstrap_node.js:394:7) at startup (bootstrap_node.js:149:9) at bootstrap_node.js:509:3

As you can see, my code is same with the node doc, and yet the result not. Please help me with the tiny question.,

Where did you see that? I can't find what you claim in the official documentation. — E_net4
– E_net4, Commented Oct 13, 2016 at 14:21
@E_net4 nodejs.org/dist/latest-v4.x/docs/api/console.html that you can Ctr+F find — Courage
– Courage, Commented Oct 13, 2016 at 14:29

E_net4 · Accepted Answer · 2016-10-13 14:52:58Z

console.error(new Error('Whoops, something bad happened'));
// Prints: [Error: Whoops, something bad happened], to stderr

This is not to be interpreted in the literal sense. Not in the latest LTS and stable versions anyway. Printing an error like that will actually print a textual representation of the error object, which was referred as [Error: Whoops, something bad happened] in the documentation. The actual intended behaviour is further clarified in the documentation of Console.error():

If formatting elements (e.g. %d) are not found in the first string then util.inspect() is called on each argument and the resulting string values are concatenated.

On the side of util.inspect(), this method "returns a string representation of object that is primarily useful for debugging". For objects of type Error, this will yield a string containing the error's message and stack trace.

> const txt = util.inspect(new Error("I'm on SO"))
undefined
> txt
'Error: I\'m on SO\n    at repl:1:26\n    at sigintHandlersWrap (vm.js:22:35)\n    at sigintHandlersWrap (vm.js:96:12)\n    at ContextifyScript.Script.runInThisContext (vm.js:21:12)\n    at REPLServer.defaultEval (repl.js:313:29)\n    at bound (domain.js:280:14)\n    at REPLServer.runBound [as eval] (domain.js:293:12)\n    at REPLServer.<anonymous> (repl.js:513:10)\n    at emitOne (events.js:101:20)\n    at REPLServer.emit (events.js:188:7)'
> console.log(txt)
Error: I'm on SO
    at repl:1:26
    at sigintHandlersWrap (vm.js:22:35)
    at sigintHandlersWrap (vm.js:96:12)
    at ContextifyScript.Script.runInThisContext (vm.js:21:12)
    at REPLServer.defaultEval (repl.js:313:29)
    at bound (domain.js:280:14)
    at REPLServer.runBound [as eval] (domain.js:293:12)
    at REPLServer.<anonymous> (repl.js:513:10)
    at emitOne (events.js:101:20)
    at REPLServer.emit (events.js:188:7)

@suoyong No problem, your question was quite reasonable (also don't forget to accept an answer). I wonder if the devs over at the Node.js foundation know about this.

robertklep · Accepted Answer · 2016-10-13 14:46:53Z

0

I guess that this changed between Node v4 and Node v6.

With v4, the output is as documented; with v6, a stack trace is included in the output, like what you're seeing.

You can work around it by using console.error(err.toString()), which for both versions will output Error: a (so minus any brackets, but if you really want those you can add them, of course).

answered Oct 13, 2016 at 14:46

robertklep

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1 Comment

E_net4 Over a year ago

The documentation in v6.8.0 shows the same thing. Now, it's hard to guess whether this was an old behaviour not updated in the documentation or just a means of explaining that the error object would be printed to stderr.

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