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keyof ignores constraint #44985
Description
Bug Report
🔎 Search Terms
keyof, generic constraint
🕗 Version & Regression Information
- This is a crash ❌ No
- This changed between versions _______ and _______
- This is the behavior in every version I tried, and I reviewed the FAQ for entries about keyof
- I was unable to test this on prior versions because it seems to be consistent behavior in all tested versions
⏯ Playground Link
Playground link with relevant code
💻 Code
// Implementation of event emitter - sample without all methods
type Listener<Event> = (event: Event) => void;
// Here the "key: K" complains "An index signature parameter type must be either 'string' or 'number'"
type Listeners<Events, K extends string & keyof Events> = {[key: K]: Listener<Events[K]>[]}
class EventEmitter<Events extends {[key: string]: unknown}> {
#listeners: Listeners<Events, keyof Events>
constructor() {
this.#listeners = {}
}
}
// Usage
type MyEvents = {
connect: void
reconnect: number
error: Error
disconnect: void
}
const emitter = new EventEmitter<MyEvents>()🙁 Actual behavior
I am getting ts errors even though the types work as expected. See // usage section
🙂 Expected behavior
The keyof returns the same type as the constraint generic and thus will be valid as index signature.
Activity
MartinJohns commentedon Jul 12, 2021
Having
keyof Tas an indexer type would require #26797.alesmenzel commentedon Jul 12, 2021
So, does that mean it is currently not possible to do?
btw. why is
K extends string & keyof Eventsnot enough to make it an indexable type since it requires 'string' or 'number' and the generic K must be a string and also keyof Events?MartinJohns commentedon Jul 12, 2021
The type
string & keyof Eventsis the same askeyof Events. And a union of keys can not be an index signature. You can use a mapped type instead.RyanCavanaugh commentedon Jul 12, 2021
This works without issue
alesmenzel commentedon Jul 12, 2021
Thanks guys,
the error message wasnt clear to me, but know I understand that you cant use a union of types which you get from the keyof - e.g. type union = 'a' | 'b' | 'c' to index but instead use it in either Record which handles the union case or use mapped type like
{[key in K]: ... }instead of{[key: K]:Lastly, do you know why there is the last error at the
.push(listener)line in.on()method ts playgroundit seems to me that those types should be the same, no? One is
Events[keyof Events](so basically any values of the Events) and the second one is the concrete eventEvents[Type]where the type is defined as constraint ofkeyof Eventsalesmenzel commentedon Jul 12, 2021
Looks like I had a mistake in the mapped type, working version for anyone interested
ts playground
Thanks Martin Johns (@MartinJohns) and Ryan Cavanaugh (@RyanCavanaugh)