Type predicate callbacks

[roobscoob]

Suggestion

🔍 Search Terms

"type predicate"

✅ 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

The following code does not function, though it can be expected to.

class ExtendedType {
    isExtended: this is ExtendedType { return true }
}

class BaseType {
    isExtended: this is ExtendedType { return false }
}

declare const array: BaseType[]

function takesCb<T extends BaseType>(cb: (arg: BaseType) => arg is T): T {
    for (let item of array) {
        if (cb(item)) return item;
    }
}

const extended = takesCb(t => t.isExtended());

// extended is expected to be of type ExtendedType

📃 Motivating Example

I'm building a client for a server, the client emits packets when received.
I have a base ClientBoundPacket class, which other packets extend.
all ClientBoundPackets have methods that look like packet.isKeepAlive() for easily checking and casting their type.
A useful pattern is being able to call an async function and have it resolve when it receives a packet that passes a boolean check.
(e.g. check for a keep alive packet):

const packet = client.waitForPacket(packet => packet.isKeepAlive());

// packet should be of type ClientBoundKeepAlivePacket (packet.isKeepAlive() returns this is ClientBoundKeepAlivePacket)

💻 Use Cases

I wish for this to be implemented to extend the power of an already awesome tool, type predicates.

[roobscoob]

To go into more detail here (re-reading, i noticed it was missed by accident) the core issue here is that in this code sample:

let cb = (t) => t.isXyz()

cb does not share the return type of t.isXyz (that is, this is Xyz) but rather boolean, which I feel is unexpected.

[MartinJohns]

Duplicate of #38390.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.