JSON.stringify returns {} even with replacer

Question

My code (fiddle) is:

let count = 0;
replacer = (k, v) => {
  console.log(`count: ${count++}`);
  if (v === undefined)
    return null;
  else
    return v;
}

try {
  const x = y;
} catch (err) {
  console.log(err);
  console.log("=====");
  console.log(`typeof err is ${typeof err}`);
  console.log(JSON.stringify(err));
  console.log("-----");
  console.log(JSON.stringify(err, replacer));
}

The output is:

ReferenceError: y is not defined
    at window.onload ((index):42)
=====
typeof err is object
{}
-----
count: 0
{}

Is err really {}? If not, how do I get a JSON string representation of it?

What browser are you using? I'm getting {"line":42,"column":18,"sourceURL":"https://fiddle.jshell.net/OldGeezer/w2gzkntv/3/show/"} in Safari. — Gerardo Furtado
– Gerardo Furtado, Commented Jul 18, 2019 at 4:09
Chrome. @Eddie converted the post to a snippet, but the result output is not the same as the Fiddle's. The snippet gives {} for console.log(err). — u936293
– u936293, Commented Jul 18, 2019 at 4:15
It may be that the error object itself has no enumerable properties, they may be on its [[Prototype]], or they might be getters (so are functions and ignored by JSON.stringify). Try for..in. — RobG
– RobG, Commented Jul 18, 2019 at 4:20
Chrome, Edge, FF and Opera are showing {}, but Safari is showing the expected result, as I said before in my comment. Therefore, this seems to be just a browser implementation difference. — Gerardo Furtado
– Gerardo Furtado, Commented Jul 18, 2019 at 4:31

RobG · Accepted Answer · 2019-07-18 05:44:10Z

1

It seems that the properties of error instances are inherited from Error.prototype and are not own properties, see ECMA-262 §19.5.4.

Safari has additional properties on the instance itself (line, column, sourceURL), however name and message are on the prototype.

You can convert it to JSON with your own function, e.g.

// Create plain object, copy error name and message properties.
// Copy own properties of error instance, then JSON.stringify it
function errToJSON(err) {
  let props = Object.assign({name:err.name, message:err.message}, err);
  return JSON.stringify(props);
}

try {
  var x = y;
} catch (e) {
  console.log('JSON.stringify(e):\n' + JSON.stringify(e));
  console.log('errToJSON(e):\n' + errToJSON(e));
  console.log('e.toString():\n' + e.toString());
}

edited Jul 18, 2019 at 5:44

answered Jul 18, 2019 at 4:39

RobG

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tiagolisalves · Accepted Answer · 2019-07-18 04:25:24Z

0

In this case err doesn't have any own enumerable properties. Only its prototype ReferenceError. You can inspect err by its inherited properties, like described in MDN

answered Jul 18, 2019 at 4:25

tiagolisalves

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Gohel Dhaval · Accepted Answer · 2019-07-18 04:54:46Z

0

It seems that the properties of error instances are inherited from Error.prototype and are not own properties, see ECMA-262 §19.5.4.
JSON.stringify return {} because the err object is not pure object there is no key value paired. so it would return {}

answered Jul 18, 2019 at 4:54

Gohel Dhaval

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