conditional-type-checks
v1.0.6
Published
Types for testing TypeScript types.
Downloads
81,830
Readme
Conditional Type Checks
As TypeScript's type system becomes more complex, it's useful to be able to write tests for what a type should be.
This library offers reusable conditional types to help test your types.
Type Checks
These will resolve to the type true
when they match and false
otherwise.
IsNullable<T>
- Checks ifT
is possiblynull
orundefined
.IsExact<T, U>
- Checks ifT
exactly matchesU
.Has<T, U>
- Checks ifT
hasU
.NotHas<T, U>
- Checks ifT
does not haveU
.IsAny<T>
- Checks ifT
is theany
type.IsNever<T>
- Checks ifT
is thenever
type.IsUnknown<T>
- Checks ifT
is theunknown
type.- More to come...
Ways to Test
Use what you prefer:
- The
AssertTrue
,AssertFalse
, orAssert
types. - The
assert
function.
Use with AssertTrue
, AssertFalse
, and Assert
Doing a test:
import type {
AssertFalse,
AssertTrue,
Has,
IsNever,
IsNullable,
} from "https://deno.land/x/conditional_type_checks/mod.ts";
const result = someFunction(someArg);
type _test =
| AssertTrue<Has<typeof result, string> | IsNullable<typeof result>>
| AssertFalse<IsNever<typeof result>>
| Assert<Has<typeof result, number>, true>;
Warning: Do not use an intersection type between checks (ex. Has<string | number, string> & IsNever<never>
) because it will cause everything to pass if only one of the checks passes.
Use with assert
Doing a test:
import {
assert,
IsExact,
} from "https://deno.land/x/conditional_type_checks/mod.ts";
const result = someFunction(someArg);
// compile error if the type of `result` is not exactly `string | number`
assert<IsExact<typeof result, string | number>>(true);
Failure:
// causes a compile error that `true` is not assignable to `false`
assert<IsNullable<string>>(true); // string is not nullable
npm Install
npm install --save-dev conditional-type-checks