[Feature] allow use `xxx is Type` with async function

Search Terms

Suggestion

Use Cases

Examples

export async function checkIPFS(ipfs): awaited ipfs is IIPFSPromiseApi
{
return true
}

if (await checkIPFS(ipfs))
{
// here will know ipfs is IIPFSPromiseApi
}

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.

Start with the TypeScript example in the issue and compare its requested async type-predicate behavior with current compiler behavior. Review the linked TypeScript design goals before deciding whether the syntax and control-flow narrowing are viable; done means the proposal has a defined, accepted behavior and tests covering the example.