What is a Promise<{}>?

Question

I want to create a promise which settles when a WebSocket connects.

import * as WebSocket from "ws"

let ws = WebSocket("ws://example.com")

function foo(echo: string) {
    return new Promise((resolve, reject) => ws.once("open", () => resolve(echo)));
}

In VS Code, the foo function infers as yielding Promise<{}>.

1. What a Promise<{}>?
2. More generally, how do I get this to refer a Promise<string>? Am I misunderstanding Trigger Promise when an event fires ?

Answer 1

Generics have {} (empty object) as default parameter value.

Promise is a generic. Since generic type cannot be inferred from resolve argument due to TypeScript limitations, it's {}, or an empty object, by default.

It should be:

function foo(echo: string) {
    return new Promise<string>((resolve, reject) => ws.once("open", () => resolve(echo)));
}