Chris Hager
Programming, Technology & More

Custom Errors in TypeScript 2.1

TypeScript 2.1 introduced a number of breaking changes, among them that “Extending built-ins like Error, Array, and Map may no longer work”.

For a subclass like the following:

class FooError extends Error {
    constructor(m: string) {
    sayHello() {
        return "hello " + this.message;
  • methods may be undefined on objects returned by constructing these subclasses, so calling sayHello will result in an error.
  • instanceof will be broken between instances of the subclass and their instances, so (new FooError()) instanceof FooError will return false.

The suggested solution of manually adjusting the prototype with Object.setPrototypeOf(this, FooError.prototype) might not work due to Object.setPrototypeOf(..) being missing.

The Solution

After trying several approaches, the solution presented here seems to be the way which reliably works, including with promises (native and bluebird):

import * as util from "util";

// Custom Error 1
const CustomError1 = function(message) {
    Error.captureStackTrace(this, this.constructor); =;
    this.message = message;
util.inherits(CustomError1, Error);

// Custom Error 2
const CustomError2 = function(message) {
    Error.captureStackTrace(this, this.constructor); =;
    this.message = message;
util.inherits(CustomError2, Error);

// Tests
let e = new CustomError1("Foo");
console.log("instanceof error: ", e instanceof Error);  // true
console.log("instanceof own class: ", e instanceof CustomError1);  // true
console.log("instanceof other error class: ", e instanceof CustomError2);  // false

If you have suggestions or feedback, let me know via @metachris.

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Posted by Chris Hager in #nodejs, #typescript, #javascript  ·  06 Jan 2017