How to limit keyof T to just string keys?

Question

I'm trying to extend existing interface:

type ColDef = { field: string; }

so that I will limit field values only to actual properties of specified type:

interface TypeSafeColDef<T> extends ColDef {
   field: keyof T
}

but I'm getting:

Interface 'TypeSafeColDef' incorrectly extends interface 'ColDef'. Types of property 'field' are incompatible. Type 'keyof TRow | undefined' is not assignable to type 'string | undefined'. Type 'keyof TRow' is not assignable to type 'string | undefined'. Type 'string | number | symbol' is not assignable to type 'string | undefined'. Type 'number' is not assignable to type 'string | undefined'. Type 'keyof TRow' is not assignable to type 'string'. Type 'string | number | symbol' is not assignable to type 'string'. Type 'number' is not assignable to type 'string'

I've tried following constraint, but no success

type StringKey = { [key: string]: any }

interface TypeSageColDef<TRow extends StringKey>

extends { [key: string]: any } behaves same as extends object for some reason (not limiting keys to be string only). Real question is why do you need extends ColDef? Do you have more properties in real example? — Aleksey L.
– Aleksey L., Commented Jan 23, 2020 at 12:36
I need to extend ColDef because I want to get compiler error when invalid field is passed. I do not own ColDef, it is a 3rd party type definition. — Liero
– Liero, Commented Jan 23, 2020 at 12:44

ford04 · Accepted Answer · 2020-01-23 14:55:16Z

16

Your last one comes quite close, we still need to extend ColDef and extract a string key type for field:

type ColDef = { field: string; }

interface TypeSafeColDef<T extends object> extends ColDef {
  field: Extract<keyof T, string>
}

// test
type T1 = TypeSafeColDef<{ a: string }> // { field: "a"; }

field now has type Extract<keyof T, string>, because keyof supports string | number | symbol property names since TS 2.9. There is a compiler option --keyofStringsOnly to disable this new behavior, if you don't want that.

A second alternative is to define a type alias to get rid of Extract. This works due to the nature of the intersection operator never provoking errors:

type TypeSafeColDefAlias<T extends object> = ColDef & {
  field: keyof T
}

Code sample

edited Jan 23, 2020 at 14:55

answered Jan 23, 2020 at 9:59

ford04

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4 Comments

Aleksey L. Over a year ago

extends StringKey looks redundant if you're anyway doing Extract<keyof T, string>

Liero Over a year ago

yeah, StringKey is not needed in this case. I suggest to edit the answer and I will accept it

ford04 Over a year ago

@Liero updated answer (and gave another possible alternative)

Victor Zakharov Over a year ago

I needed to enforce string type for grid column helper. Here is how I did it export class GridColumnHelper<T> { ... createColumn(field: Extract<keyof T, string> ... }. This is instead of field: keyof T. Hope it helps someone.

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