CODEWITHSUNDEEP
javascripttypescriptoopinheritanceinstanceof
supergentle
supergentle

Reputation: 1091

Why instanceof returns false for a child object in Javascript

I have the Child class that extends Parent class. So let say I created a new instance "child" out of the Child class. When I check the condition child instanceof Child, it returns false. However, child instanceof Parent returns true.

Why does this so?

EDIT

So I found this only happens when I extend the Child class with Error class. Let me leave the code example below.

class Child extends Error {
  constructor(message) {
    super(message);
  }
}
const ch = new Child();
console.log(ch instanceof Child);

2nd EDIT

class PullCreditError extends Error {
  public name: string;
  public message: string;
  public attemptsRemaining: number;
  constructor(message: string, attemptsRemaining: number) {
    super();
    Error.captureStackTrace(this, PullCreditError);
    this.name = 'PullCreditError';
    this.message = message;
    this.attemptsRemaining = attemptsRemaining;
  }
}

Upvotes: 8

Views: 4112

Answers (1)

CertainPerformance
CertainPerformance

Reputation: 370679

This is a documented bug:

https://github.com/Microsoft/TypeScript/issues/15875

Extending built-ins like Error, Array, and Map may no longer work

As part of substituting the value of this with the value returned by a super(...) call, subclassing Error, Array, and others may no longer work as expected. This is due to the fact that constructor functions for Error, Array, and the like use ECMAScript 6's new.target to adjust the prototype chain; however, there is no way to ensure a value for new.target when invoking a constructor in ECMAScript 5. Other downlevel compilers generally have the same limitation by default.

The recommendation is to adjust the prototype manually using setPrototypeOf in the constructor. A fix for your PullCreditError class would look like this:

export class PullCreditError extends Error {
  public name: string;
  public message: string;
  public attemptsRemaining: number;
  constructor(message: string, attemptsRemaining: number) {
    super();
    Object.setPrototypeOf(this, PullCreditError.prototype); // <-------
    Error.captureStackTrace(this, PullCreditError);
    this.name = 'PullCreditError';
    this.message = message;
    this.attemptsRemaining = attemptsRemaining;
  }
}

Upvotes: 13

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