NodeJS/Javascript: how to detect an error object return value?

Question

I have a function that might return some object or might return a custom error object. I am unable to detect error object type.

I have tried constructor.match(/Error/i) or tried to work on Object.keys of prototype but nothing worked. Following is the code?

function x() {
    try {
        ...
    } catch {e} {
       let err = new Error('caught error')
       return err
    }
    return someObject
}

//the following gives: TypeError: err.constructor.match is not a function
if (x().constructor.match(/Error/i)) {
   //log error
}
//do something

Any ideas how to detect the output error type?

@JaydipJ: function x () {let err = new Error(); return err} console.log(JSON.stringify(x()).includes('error')) --> gives false — Ulysses
– Ulysses, Commented May 29, 2017 at 7:17
"".include() returns true false as per the condition. you could use this in if block — Jaydip Jadhav
– Jaydip Jadhav, Commented May 29, 2017 at 7:19

Vibhanshu · Accepted Answer · 2017-05-29 07:24:50Z

9

You can check if returned object is an instanceof Error as follows

let y = x();
if(y instanceof Error) {
  // returned object is error
} else {
 //returned object is not error
}

answered May 29, 2017 at 7:24

Vibhanshu

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2 Comments

Kirill Rogovoy Over a year ago

Other options are if (Error.prototype.isPrototypeOf(y)) or duck-typing like if (y.stack !== undefined)

Ulysses Over a year ago

@thanks Kiril Rogovoy - that works too but the one in the solution is but more readable

Paul D. Waite · Accepted Answer · 2019-06-07 15:07:58Z

0

Try this:

All Error instances and instances of non-generic errors inherit from Error.prototype. As with all constructor functions, you can use the prototype of the constructor to add properties or methods to all instances created with that constructor.

Standard properties:

Error.prototype.constructor: Specifies the function that created an instance's prototype.
Error.prototype.message: Error message
Error.prototype.name: Error name

try {
  throw new Error('Whoops!');
} catch (e) {
  console.log(e.name + ': ' + e.message);
}

edited Jun 7, 2019 at 15:07

Paul D. Waite

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answered May 29, 2017 at 7:17

Francesco Taioli

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2 Comments

Ulysses Over a year ago

function x () {let err = new Error("err"); return err} console.log(x().prototype) <-- gives undefined Did you mean this?

Francesco Taioli Over a year ago

check this developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/…

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