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.

Reproduce the linked Playground examples, especially the assertion involving a typed index signature and an extra property. Start by locating TypeScript's type-assertion compatibility checks and their regression tests; done means the proposed assertion succeeds while incompatible assertions continue to be rejected.