keyof {[n: number]: any} should be string, not never #22105
Description
TypeScript Version: 2.8.0-dev.20180216
Search Terms:
- keyof
- numeric indexer
Code
// Behaviour with string keys
type StringKeys1 = {a: any, b: any, c: any}; // Specific keys only
type StringKeys2 = {[s: string]: any}; // Indexer only
type StringKeys3 = {[s: string]: any, foo: any}; // Indexer + some keys
type S1 = keyof StringKeys1; // S1 = "a" | "b" | "c"
type S2 = keyof StringKeys2; // S2 = string
type S3 = keyof StringKeys3; // S3 = string
// Behaviour with numeric keys
type NumberKeys1 = {1: any, 2: any, 3: any}; // Specific keys only
type NumberKeys2 = {[n: number]: any}; // Indexer only
type NumberKeys3 = {[n: number]: any, 42: any}; // Indexer + some keys
type N1 = keyof NumberKeys1; // N1 = "1" | "2" | "3"
type N2 = keyof NumberKeys2; // N2 = never <===== should be string
type N3 = keyof NumberKeys3; // N3 = "42" <===== should be stringExpected behavior:
N2 and N3 should be string.
Actual behavior:
N2 is never and N3 is "42".
Related Issues:
#13715 seems related but isn't really - it is asking for keyof T to allow numbers.
#22042 has an example showing how lodash typings incorrectly produce never for objects that have numeric indexers.
Discussion:
In JavaScript, both string and numeric keys end up effectively being string keys. keyof uniformly handles string keys, numeric keys, and string indexers. Why does it ignore numeric indexers?