Writing Type Tests

Assertions

Import from tspub/test-d:

import { expectType, expectNotType, expectAssignable, expectNever, expectAny } from "tspub/test-d";

expectType<T>(value)

Assert that value is exactly type T:

import { expectType } from "tspub/test-d";
import { myFn } from "../dist/index.js";

expectType<string>(myFn("hello"));

expectNotType<T>(value)

Use with @ts-expect-error to assert a type mismatch:

import { expectNotType } from "tspub/test-d";

// @ts-expect-error — string is not number
expectNotType<number>("hello");

expectAssignable<T>(value)

Assert that value is assignable to type T (allows subtypes):

import { expectAssignable } from "tspub/test-d";

expectAssignable<{ name: string }>({ name: "test", extra: true });

expectNever<T>(value)

Assert that a type is never:

import { expectNever } from "tspub/test-d";

type Result = Extract<string, number>; // never
expectNever<Result>(null as never);

expectAny<T>(value)

Assert that a type is any:

import { expectAny } from "tspub/test-d";

declare const x: any;
expectAny(x);

Testing Compile Errors

Use @ts-expect-error to verify that invalid usage causes a type error:

import { myFn } from "../dist/index.js";

// @ts-expect-error — should not accept number
myFn(42);

// @ts-expect-error — missing required argument
myFn();

If the line below @ts-expect-error does NOT cause an error, TypeScript will report an "unused @ts-expect-error" error — which is what we want.

Custom Directory

tspub test-types --dir my-type-tests

Or in config:

export default {
  check: {
    typeTests: {
      enabled: true,
      directory: "my-type-tests",
    },
  },
};