Allow type assertions to consider typed index signatures

Search Terms

interface, index, signature, typed, assertion, conversion, partial,

Suggestion

Currently type assertions do not seem to respect typed index signatures [key: string]: any; in interfaces or test for compatibility before attempting to convert.

I am proposing that type assertions would be able to convert this situation without error:

interface Test {
	a: string;
	b?: string;
	[key: string]: any;
}

const a = {b: 'b', c: 3 } as Test;

Use Cases

Currently to work around this I need to first create an intermediate variable that is typed as a Partial before making the assertion. I would prefer to just make the assertion.

Examples

This example above will error because {b: 'b', c: 3 } is missing property a

interface Test {
	a: string;
	b?: string;
	[key: string]: any;
}

const a = {b: 'b', c: 3 } as Test;

playground

If I remove c: 3 from the object then there is no error and a is no longer required:

interface Test {
	a: string;
	b?: string;
	[key: string]: any;
}

const a = {b: 'b' } as Test;

playground

Currently to workaround this an intermediate typed value can be used so the subtype is already confirmed before the assertion:

interface Test {
	a: string;
	b?: string;
	[key: string]: any;
}

const test: Partial<Test> = { b: 'b', c: 3 };
const a = test as Test

playground

Checklist

My suggestion meets these guidelines:

• This wouldn't be a breaking change in existing TypeScript/JavaScript code
• This wouldn't change the runtime behavior of existing JavaScript code
• This could be implemented without emitting different JS based on the types of the expressions
• This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

リンクされた Playground の例を再現し、特に型付きインデックスシグネチャと追加のプロパティに関するアサーションを再現します。まず、TypeScript の型アサーションの互換性チェックとそのリグレッションテストを見つけます。提案されたアサーションが成功し、互換性のないアサーションが引き続き拒否されれば完了です。