TypeScript TV

TS2361

The right-hand side of an 'in' expression must not be a primitive.

Broken Code ❌

import axios, { AxiosError } from 'axios';
 
const isAxiosError = (error: unknown): error is AxiosError => {
  return 'isAxiosError' in error;
};

Fixed Code ✔️

You cannot use the in keyword on primitive data types. That's why we have to replace the primitive type unknown with a non-primitive type like object:

import axios, { AxiosError } from 'axios';
 
const isAxiosError = (error: object): error is AxiosError => {
  return 'isAxiosError' in error;
};

An alternative solution would be to use a type assertion on the right-hand side of the 'in' expression:

const isAxiosError = (error: unknown): error is AxiosError => {
  return 'isAxiosError' in (error as AxiosError);
};