Why need to assign constructor back to my custom error type in JavaScript [duplicate]

Question

I am learning "Error Handling in JavaScript" and read articles on site point. In the articles, it shows the way to make custom error type:

function DivisionByZeroError(message) {
  this.name = "DivisionByZeroError";
  this.message = (message || "");
}

DivisionByZeroError.prototype = new Error();
DivisionByZeroError.prototype.constructor = DivisionByZeroError;

However, I can not understand why the function constructor has to assign to itself again. That is,

DivisionByZeroError.prototype.constructor = DivisionByZeroError;

I think this is because DivisionByZeroError.prototype assigned to new Error() so the prototype was changed.

But I am not sure why even after commenting out that line. The code still works fine.

Demo code is put on jsfiddle

Reference:
Exceptional Exception Handling in JavaScript
A Guide to Proper Error Handling in JavaScript

nodeway.wordpress.com/2015/02/16/javascript-prototype this might be useful to understand — sandyJoshi
– sandyJoshi, Commented Dec 6, 2017 at 8:40
Avoid that code, using new for inheritance is horrible. Subclassing builtins like Error works best with ES6 class. — Bergi
– Bergi, Commented Dec 6, 2017 at 8:50

notrota · Accepted Answer · 2017-12-06 08:32:11Z

The reason behind this, is that if you run the following code:

console.log(new Error().constructor)

it outputs:

"ƒ Error() { [native code] }"

Now, by standard, if you create a class, then the following should be true:

console.log(new MyClassName().constructor === MyClassName)

for example, the following is true:

(new Error().constructor) === Error; // true

A use for this could be like so:

function isArray(maybeArray) {
  return maybeArray.constructor === Array;
}
isArray([]); // true
isArray(new Array(10)); // true
isArray(false); // false

// or say this function
function makeAnotherOfObjectWithoutParameters(object) {
    return new object.constructor();
}
(makeAnotherOfObjectWithoutParameters([])); // empty array

So now say you want to see if an object is a DivisionByZeroError. You have your function like so:

function DivisionByZeroError(message) {
  this.name = "DivisionByZeroError";
  this.message = (message || "");
}

DivisionByZeroError.prototype = new Error();

now, you should expect the following to be true:

console.log(new DivisionByZeroError('my message').constructor === DivisionByZeroError);

but you would be surprised that it is false, so now you have to explicitly set the constructor like so:

DivisionByZeroError.prototype.constructor = DivisionByZeroError

and now the above code will work!

Hope this helped someone... I learnt something too!

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