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beetcb/util.ts

Last active August 18, 2023 09:30
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Exclude number type from typescript `keyof` string indexed signature
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util.ts
// see -> https://www.typescriptlang.org/docs/handbook/2/keyof-types.html
type ExcludeUnexpectedNumIdxSigKeyof<T> = keyof T extends number
? keyof T
: Extract<keyof T, string>;
type MapishMixed = { [k: number | string]: boolean };
type M = ExcludeUnexpectedNumIdxSigKeyof<MapishMixed>;
// ^? M will be of type string | number
type MapishNumber = { [k: number]: boolean };
type N = ExcludeUnexpectedNumIdxSigKeyof<MapishNumber>;
// ^? N will be of type number
type MapishString = { [k: string]: boolean };
type O = ExcludeUnexpectedNumIdxSigKeyof<MapishString>;
// ^? O will be of type string
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