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?

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?

• javascript
• node.js

• JSON.stringify(x()).includes('error') ?? – Jaydip Jadhav Commented May 29, 2017 at 7:15
• 2 x instanceof Error – Sirko Commented May 29, 2017 at 7:16
• @JaydipJ: function x () {let err = new Error(); return err} console.log(JSON.stringify(x()).includes('error')) --> gives false – 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 Commented May 29, 2017 at 7:19
• @JaydipJ: Could you please elaborate – Ulysses Commented May 29, 2017 at 7:23

2 Answers 2

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
}

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);
}