Generics are lost during `Function.prototype.bind()` and `Function.prototype.call()`

Suggestion

🔍 Search Terms

generic, bind, call

✅ Viability 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, new syntax sugar for JS, etc.)
• This feature would agree with the rest of TypeScript's Design Goals.

⭐ Suggestion

Currently, binding or calling a function loses its generics. Suggestion: Don't lose the generic.

📃 Motivating Example

Let's say I have a generic function getMediaAd

export async function getMediaAd<T extends Ad>(
	this: Client,
	params: { locale?: string; media_id: string; format: T['format'] },
) {
	const url = this.cmsUrl(`api/ads/random`, params)
	if (!params.locale) url.searchParams.set('locale', this.locale)
	return this.get<T>(url, this.cmsHeaders())
}

This function can obtain multiple types of ad formats, determined by T.

I want to put this function inside a Client class to access some client-specific features.

export class Client {
	getMediaAd = getMediaAd.bind(this)
}

However, when I do that, this.getMediaAd loses its generic parameter.

Losing the generic parameter means I can't tell it what is T supposed to be, like this:

export async function getMediaBillboardAd(
	this: Client,
	params: { locale?: Intl.UnicodeBCP47LocaleIdentifier; media_id: string },
) {
	return this.getMediaAd<AdBillboard>({ ...params, format: adFormat.billboard })
}

Here, T is AdBillboard. However, getMediaAd expects 0 generic parameters.

💻 Use Cases

I want to use this to create an API wrapper client, but keep the ability to declare functions in different files.

Inizia con l’esempio generico motivante getMediaAd e l’assegnazione di Client.getMediaAd usando Function.prototype.bind(). Confronta il comportamento dichiarato di bind() e call() con il sistema di tipi corrente di TypeScript, quindi definisci come preservare i parametri generici senza modificare il comportamento di JavaScript a runtime.