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>>;
                                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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:

• static-type-assert
• ts-expect

Other utility types libraries:

• dtslint
• tsd-check
• type-plus