error TS2677: Type 'keyof E' is not assignable to type 'string'

TypeScript Version: Version 2.9.0-dev.20180426

Search Terms: keyof not assignable string

Code

//Just a minimal example, not what I am actually doing
function isKey<E> (str : string) : str is keyof E {
    return true;
}

Expected behavior:

Should compile fine. The body of the type guard may actually correctly assert that str is, indeed, keyof E

Works fine on TypeScript 2.8.3

Actual behavior:

error TS2677: A type predicate's type must be assignable to its parameter's type.
  Type 'keyof E' is not assignable to type 'string'.
    Type 'string | number | symbol' is not assignable to type 'string'.
      Type 'number' is not assignable to type 'string'

Playground Link: Works on the playground

Related Issues:

This is a breaking change in TS 2.9 (documentation should come soon) introduced by #23592. With #23592 keyof now returns all known keys of a types including string, number and symbol keys. and as such, the constraint on keyof E is now string | number | symbol.

If you are interested in getting only the string portions of the keys, use Extract:

function isKey<E>(str: string): str is Extract<keyof E, string> {
    return true;
}

if your function is meant to handle all property names, including symbols, then consider widening the declaration of str:

function isKey<E> (str : string | number | symbol) : str is keyof E {
    return true;
}

Also, as a temporary measure you can use the new --keyofStringsOnly compiler option to revert to the old behavior.

Thanks for the swift response!

Fix TypeScript 2.9 build

Change tsconfig.json to be able to execute tsc in watch mode

However, if I specifically mention that my type has only string keys like this:

interface A { [k: string]: any; }
type b = keyof A;

still, I get the type of b as string | number as seen in the IntelliSense on vs code:

Why is that so? Why is it not string only?
Typescript used: 3.9.3

For typescript 4, I use this in place of keyof as a current workaround to get accurate key types

type KeyTypes<T> = {
    [K in keyof T]-?: K extends string ?
        string :
        K extends number ?
            number :
            K extends symbol ?
                symbol :
                never
}[keyof T];

type KeyOfType<T, KeyType extends string | number | symbol = KeyTypes<T>> = Extract<keyof T, KeyType>;

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error TS2677: Type 'keyof E' is not assignable to type 'string' #23724

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error TS2677: Type 'keyof E' is not assignable to type 'string' #23724

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mhegazy commented on Apr 26, 2018

ahejlsberg commented on Apr 26, 2018

AnyhowStep commented on Apr 27, 2018

abdatta commented on Jun 4, 2020

SLGShark6 commented on Jan 30, 2021

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