How to specify a "caused by" in a JavaScript Error?

Question

In my NodeJS program, I parse some user JSON file.

So I use :

this.config = JSON.parse(fs.readFileSync(path));

The problem is that if the json file is not correctly formated, the error thrown is like:

undefined:55
            },
            ^
SyntaxError: Unexpected token }
    at Object.parse (native)
    at new MyApp (/path/to/docker/lib/node_modules/myApp/lib/my-app.js:30:28)
...

As it is not really user friendly I would like to throw an Error specifying some user friendly message (like "your config file is not well formated") but I want to keep the stacktrace in order to point to the problematic line.

In the Java world I used throw new Exception("My user friendly message", catchedException) in order to have the original exception which caused that one.

How is it possible in the JS world?

Yes this is how I wanted to do at first. But I wanted to keep the original stack also, which contains the original message pointing to the problematic line (in my example)... And I can't find a "good" way to do that in the JS error handling I saw until now on the web... — Anthony O., Sep 10 '14 at 11:13
I actually see that my question is a duplicate of: http://stackoverflow.com/questions/17886769/how-to-chain-exceptions-in-javascript-ie-add-cause-like-in-java — Anthony O., Sep 10 '14 at 11:44
Node.js now supports adding specific cause of the error: https://stackoverflow.com/questions/1382107/whats-a-good-way-to-extend-error-in-javascript/69365658#69365658 — t_dom93, Sep 28 '21 at 16:54

Anthony O. · Accepted Answer · 2016-09-19T15:25:14.737

39

What I finally did is:

try {
    this.config = JSON.parse(fs.readFileSync(path));
} catch(err) {
    var newErr = new Error('Problem while reading the JSON file');
    newErr.stack += '\nCaused by: '+err.stack;
    throw newErr;
}

edited Sep 19 '16 at 15:25

answered Aug 31 '15 at 09:19

Anthony O.

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And yet the ["primary source"](https://www.joyent.com/developers/node/design/errors) regarding best practices mentions not to touch the "stack" property at all. In java there's a "cause" property. Do nodejs people expect that to be set? – dmansfield Sep 14 '15 at 15:04
A better answer would be something that only calculates the stack when it is read. Something like Object.defineProperty(newErr, 'stack', {get: function() { return ??? + '\nCaused by: ' + err.stack } }). But barring that, this seems pretty nice to me. – Adam A Apr 26 '16 at 06:28
1

I assume the last line should be: `throw newErr;` – fmpdmb Sep 16 '16 at 16:55
this is what i was looking for thanks. This is a clear example of what to do when you have a custom stack trace and you are throwing an error from a controlled situation (ie when a library provides you a with a stacktrace without an error) – Illiax May 14 '18 at 21:17

Sebastien Lorber · Answer 2 · 2021-12-01T16:38:57.473

There is an new Error Cause proposal for ECMAScript, and it reached stage-4 at TC34!

It means it will be in the next ECMAScript version!

https://github.com/tc39/proposal-error-cause

You would provide the cause as an error option:

throw new Error(`Couldn't parse file at path ${filePath}`, { cause: err });

The ES proposal only formalize it on the language level, but browsers/NodeJS should normally agree to log the full causal chain in practice (see https://github.com/nodejs/node/issues/38725)

As of today (end of 2021), Firefox Devtools are already able to log nested stacktraces!

score 4 · Answer 3 · answered Oct 27 '16 at 14:42

4

Joyent released a Node.js package that can be used exactly for that. It is called VError. I paste an example of how you would use the pacakge:

var fs = require('fs');
var filename = '/nonexistent';
fs.stat(filename, function (err1) {
    var err2 = new VError(err1, 'stat "%s"', filename);
    console.error(err2.message);
});

would print the following:

stat "/nonexistent": ENOENT, stat '/nonexistent'

answered Oct 27 '16 at 14:42

eliocs

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Please don't post identical answers to [multiple questions](http://stackoverflow.com/a/40287580). Post one good answer, then vote/flag to close the other questions as duplicates. If the question is not a duplicate, *tailor your answers to the question.* – Paul Roub Oct 27 '16 at 14:56
[@netflix/nerror](https://github.com/Netflix/nerror) is a more modern alternative to verror and has built-in types. – raythurnevoid Feb 08 '21 at 11:18

score 0 · Answer 4 · answered Feb 08 '21 at 08:52

0

2021 Update: To chain exceptions in JS:

class MyAppError extends Error {
    constructor(...params) {
        super(...params)
        if (Error.captureStackTrace) {
            // This is the key line!
            Error.captureStackTrace(this, this.constructor);
        }
        this.name = this.constructor.name
    }
}

See the Mozilla docs on Error.captureStackTrace

answered Feb 08 '21 at 08:52

Erin

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This is not standard, it's a specific V8 function. – raythurnevoid Feb 08 '21 at 11:16

Moob · Answer 5 · 2014-09-10T12:22:48.657

-4

Use a try / catch block:

try {
    this.config = JSON.parse("}}junkJSON}");
    //...etc
}
catch (e) {
    //console.log(e.message);//the original error message 
    e.message = "Your config file is not well formatted.";//replace with new custom message
    console.error(e);//raise the exception in the console
    //or re-throw it without catching
    throw e;
}

http://jsfiddle.net/0ogf1jxs/5/

UPDATE: If you really feel the need for a custom error you can define your own:

function BadConfig(message) {
   this.message = message;
   this.name = "BadConfig";
}
BadConfig.prototype = new Error();
BadConfig.prototype.constructor = BadConfig;

try {
    this.config = JSON.parse("}}badJson}");
} catch(e) {
    throw new BadConfig("Your JSON is wack!");
}

http://jsfiddle.net/kL394boo/

Lots of useful info at https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error

edited Sep 10 '14 at 12:22

answered Sep 10 '14 at 11:16

Moob

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I wanted to "throw" something because I'm in library code, so I want the developer who uses it to be able to catch it and display it to the user in the way he wants... Actually I want to do something like passing the root cause to a new Exception in Java – Anthony O. Sep 10 '14 at 11:21
See: http://stackoverflow.com/questions/783818/how-do-i-create-a-custom-error-in-javascript – Moob Sep 10 '14 at 11:29
1

Just edit this one and change e.message to your user friendly message and throw e – Jonas Liljestrand Sep 10 '14 at 11:44
@JonasLiljestrand - Updated answer accordingly – Moob Sep 10 '14 at 11:53
1

Thank you for your answer but it still doesn't reply to my question which is: how to chain exceptions in JS like adding cause to an Exception in Java. Your `BadConfig` class doesn't include the catched `e` and can't display it as a cause in the error log if the one who catch the `BadConfig` output it in `console.error`. – Anthony O. Sep 10 '14 at 13:26
I must be misunderstanding your requirements. The `stack` is still available. See test here: http://jsfiddle.net/kL394boo/1/ – Moob Sep 10 '14 at 14:58

How to specify a "caused by" in a JavaScript Error?

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