ts-type-assert
Type assertion library for TypeScript.
The library requires conditional types, so the minimum supported TypeScript version is 2.8.0.
Installation
$ npm i --save-dev ts-type-assert
or
$ yarn add -D ts-type-assert
Basics and motivation
Let's suppose you want to declare a type that is a union of two other types. One type is built-in and the other one is custom:
import { ComponentProps } from 'react';
type NativeButtonProps = ComponentProps<'button'>;
interface CustomButtonProps {
error?: string;
warning?: string;
}
type ButtonProps = NativeButtonProps & CustomButtonProps;How do you make sure that none of the properties from CustomButtonProps overlap those defined in NativeButtonProps? ts-type-assert is to the rescue! Just use MergePropsWithoutOverlapping utility type:
import { Assert, MergePropsWithoutOverlapping } from 'ts-type-assert';
// ...
type ButtonProps = Assert<
MergePropsWithoutOverlapping<NativeButtonProps, CustomButtonProps>
>;In case props overlap you will get a (somewhat) handy error message:
type CustomButtonProps = {
disabled?: string;
};error TS2344: Type 'AssertionError<"Some properties of T1 and T2 overlap", { __properties: "disabled"; }>' does not satisfy the constraint 'AssertionSuccess<unknown>'.
Type 'AssertionError<"Some properties of T1 and T2 overlap", { __properties: "disabled"; }>' is missing the following properties from type 'AssertionSuccess<unknown>': __value, __asserted
10 export type ButtonProps = Assert<MergePropsWithoutOverlapping<NativeButtonProps, CustomButtonProps>>;
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Other utility types
Is<T1, T2>
Checks whether T1 extends T2.
import { User } from './generated-models';
type UserId = Assert<Is<User['id'], number>>;Other packages similar to this one
These two are very similar, but require you to actually write code (call dummy assert function) instead of using plain types:
Other utility types libraries: