geoffreytools/ts-spec

A small library for testing your types

ts-spec's Issues

ts-spec no longer works in TS 4.8

The fix in #5 caused a regression in TS 4.8.

In this version, Uppercase<string> | Lowercase<string> | Capitalize<string> | Uncapitalize<string> can't be resolved to anything and any conditional type mentioning them makes the problem bubble up, to the point the entire disambiguation algorithm and the two operands of the comparison ultimately fail to resolve as well.

false positive on unions with `undefined`

import {test} from 'ts-spec'

//Type 'FailingTest<"pass", number, number | undefined>' is not assignable to type 'PassingTest | PassingTest[]'.
test('pass' as const, (t) => t.equal(1 as number | undefined)<number | undefined>())

playground

`Uppercase<string>` and `Lowercase<string>` incorrectly considered equal

import { test } from 'ts-spec'

test('' as const, (t) => t.not.equal('' as Uppercase<string>)<Lowercase<string>>()) // error

playground

feature request: assignability

I would like a way to write a test like the following:

interface Foo {
  foo: string | number;
}

test("{ foo: 10 } is assignable to Foo", (t) => t.assignable({ foo: 10 })<Foo>());

This seems to be subtly different to extends and includes in that extends tests that A is a strict superset of B instead of testing that A is sufficient to satisfy B.

optional properties incorrectly considered equal

import { test } from 'ts-spec'

test('' as const, (t) => t.not.equal({} as { a: number; b?: string })<{ a: number }>())) // error

playground

Regression in 1.4.3

The follow usage used to work in 1.4.0 and doesn't work in the current release:

class Foo {}
const foo = { foo: Foo } as const;
test("object maps to constructors", (t) => t.equal(foo)<{ readonly foo: new () => Foo }>());

I'm not sure if this is a bug or if I'm misusing the testing library but I can't make this case work out again now.

// It looks like this succeeds:
test("class includes a constructor", (t) => t.includes<Foo, new () => Foo>());
// But both of these fail:
test("class includes a constructor", (t) => t.includes(Foo)<new () => Foo>();
test("object maps to constructors", (t) => t.includes(foo)<{ foo: new () => Foo }>());

I can't seem to get anything to fail in a TS Playground

@geoffreytools i can't seem to get anything to fail in ts-spec

import { test } from 'ts-spec';

test('test description' as const, t => 
    t.equal(1)<string>() // no error
);

playground

Originally posted by @DetachHead in microsoft/TypeScript#27024 (comment)

`readonly` and mutable types are incorrectly considered equal

import { test } from 'ts-spec'

interface Foo {
    readonly a: number
}

declare const foo: { a: number }

test('' as const, (t) => t.not.equal(foo)<Foo>()) // error

playground