How to Fix instanceof Not Working For Custom Errors in TypeScript

In JavaScript, you can create custom errors by extending the built-in Error object (ever since ES 2015).

You can do the same thing in TypeScript, but there is an important caveat if your tsconfig.json has a compilation target of ES3 or ES5. In that case, instanceof doesn't work, which breaks any logic that is based on whether or not an error is a case of the custom error.

You can try this out yourself in the TypeScript playground. This is a known issue that started with TypeScript version 2.1. The recommended fix is to manually set the prototype in the constructor.

Any custom errors which further extend DatabaseError still need the same adjustment.

Upgrade the Compilation Target

Remember that this is only an issue if your compilation target is ES3 or ES5. Instead of having to remember to set the prototype, you could consider upgrading your target to ES 2015 or even later. ES 2015 has over 97% browser support, so it may be a reasonable choice for you, especially if you are okay with dropping support for Internet Explorer.

Originally published at https://www.dannyguo.com on April 17, 2021.