blog - How to Fix instanceof Not Working For Custom Errors in TypeScript

broken-instanceof.ts

class DatabaseError extends Error {}

const error = new DatabaseError("Unique constraint violation");

// prints "true"
console.log(error instanceof Error);
// incorrectly prints "false"
console.log(error instanceof DatabaseError);

custom-error.js

class DatabaseError extends Error {}

fixed-instanceof.ts

class DatabaseError extends Error {
    constructor(message: string) {
        super(message);

        Object.setPrototypeOf(this, DatabaseError.prototype);
    }
}

const error = new DatabaseError("Unique constraint violation");

// both print "true" now
console.log(error instanceof Error);
console.log(error instanceof DatabaseError);

more-extending.ts

class DatabaseError extends Error {
    constructor(message: string) {
        super(message);

        Object.setPrototypeOf(this, DatabaseError.prototype);
    }
}

class DatabaseConnectionError extends DatabaseError {
    constructor(message: string) {
        super(message);

        Object.setPrototypeOf(this, DatabaseConnectionError.prototype);
    }
}

const error = new DatabaseConnectionError("Invalid credentials");

// all print "true"
console.log(error instanceof Error);
console.log(error instanceof DatabaseError);
console.log(error instanceof DatabaseConnectionError);