bun:test - 'expect' type narrowing - How hard would it be for e.g. expect(x).toBeTrue() to narrow types?

[jalexw]

When writing tests I frequently find myself repeating myself in order to get the bun:test expect call and the TypeScript compiler on the same page...

Example:

expect(data).toBeTruthy();
// typescript compiler still thinks 'data' is potentially undefined here
if (!data) throw new Error("Data undefined");
// now tsc knows that data is undefined
// Using TypeScript 5.8.3 and Bun 1.2.11.

This makes the expect call rather redundant... It would be nice to be able to omit the if (!data) throw here in place of the expect call, as one might do in a plain JS test. I guess when writing Bun tests in TypeScript I should just throw and not bother with the expect call?

[alii]

Unfortunately this is not possible - please see #18199

A workaround I personally suggest is the following

import {expect} from 'bun:test';

export function toBeTruthy(value: unknown): asserts value {
    expect(value).toBeTruthy();
}

TypeScript only allows the assertion to exist on the function accepting the argument that is being asserted directly, rather than on a returned value (e.g. the Matchers interface which has .toBeTruthy())