Typescript: Type-Narrowing Errors

[aust1nz]

Does anyone have recommendations for type-narrowing the errors that postgres throws out, when using Typescript? Typescript's strict with errors, and treats them as unknown unless you can identify them as instances of a particular class or narrow the properties with a function.

It doesn't look like there's an exported PostgresError class, so I can't use:

if (err instanceof PostgresError)

I've settled on a type-narrowing function that feels a bit like cheating:

export function isPostgresError<T>(obj: T): obj is T & {
  name: 'PostgresError';
  code: string;
  severity_local?: string;
  severity?: string;
  position?: string;
  file?: string;
  line?: string;
  routine?: string;
  detail?: string | undefined;
  hint?: string | undefined;
  internal_position?: string | undefined;
  internal_query?: string | undefined;
  where?: string | undefined;
  schema_name?: string | undefined;
  table_name?: string | undefined;
  column_name?: string | undefined;
  data?: string | undefined;
  type_name?: string | undefined;
  constraint_name?: string | undefined;
} {
  return (
    !!obj &&
    (typeof obj == 'object' || typeof obj == 'function') &&
    'name' in obj &&
    obj.name === 'PostgresError' &&
    'code' in obj &&
    typeof obj.code === 'string'
  );
}

I can use this like so:

} catch (err) {
  if (isPostgresError(err)) {
    if (
      err.code === ...
      ...
    ) {
      // Throw appropriate user-friendly error
    }
  }

This is fine, I suppose, but I'm curious if there are any more elegant solutions. Thanks!

[aust1nz]

Doh! I saw the solution in an open issue.

import postgres from 'postgres';

try {
  const result = await sql`select * from users`
} catch (err) {
  if (err instanceof postgres.PostgresError) {
  console.log(err.code) // This is now allowed!
}

I got stumped because if you import the named PostgresError from the package, you wind up with a Typescript type that doesn't work for type narrowing.

import { PostgresError } from 'postgres';

try {
  const result = await sql`select * from users`
} catch (err) {
  if (err instanceof PostgresError) {  // This won't work! PostgresError is a Typescript type.
}