Inconsistency in inferring string indexers

[sunyatasattva]

TypeScript Version: 3.2.2
Search Terms: String index

Code

interface TestInterface {
  movX: number,
  movY: number
}

let str: "X" | "Y" = "Y";
const testObject: TestInterface = {
  movX: 0,
  movY: 0
};

testObject[`mov${str}`] *= 33;
testObject['movX'] *= 34;

Expected behavior:
TypeScript correctly recognizes that testObject[`mov${str}`] can only be type: number because str can only be type: "X" | "Y".

Actual behavior:
Typescript complains:

Element implicitly has an 'any' type because type 'TestInterface' has no index signature.

Playground Link:
http://www.typescriptlang.org/play/#src=interface%20TestInterface%20%7B%0D%0A%20%20movX%3A%20number%2C%0D%0A%20%20movY%3A%20number%0D%0A%7D%0D%0A%0D%0Alet%20str%3A%20%22X%22%20%7C%20%22Y%22%20%3D%20%22Y%22%3B%0D%0Aconst%20testObject%3A%20TestInterface%20%3D%20%7B%0D%0A%20%20movX%3A%200%2C%0D%0A%20%20movY%3A%200%0D%0A%7D%3B%0D%0A%0D%0AtestObject%5B%60mov%24%7Bstr%7D%60%5D%20*%3D%2033%3B%0D%0AtestObject%5B'movX'%5D%20*%3D%2034%3B

Also, this is just a short example, I don't want to declare a string indexer, because my real-world object is more complex than the one shown in the example.

[AnyhowStep]

Yikes. As far as I know, TS doesn't do compile-time string interpolation.

This is more a feature request than a bug report.

To TS, `mov${str}` can only ever be of type string. Would be pretty interesting to have compile-time string interpolation and resolving to string literal types.

[RyanCavanaugh]

Duplicate of #14584

[sunyatasattva]

Thank you both @RyanCavanaugh and @AnyhowStep for the reply. Both #14584, and #13969 referenced within, have not been updated in close to two years. What's the official stance on this request? This seems to be to be consistent with the way TS currently works.

Is there any workaround, to avoid having TS complain about this issue?

[milesj]

Does this work?

testObject[str === 'X' ? 'movX' : 'movY'] *= 33;

If not, you can probably force the key.

testObject[(str === 'X' ? 'movX' : 'movY') as keyof TestInterface] *= 33;

[sunyatasattva]

Thanks for the suggestion, I tried the following, and it works:

const key = `mov${str}` as keyof TestInterface;

testObject[key] *= 33;

…except that it doesn't, really. The above was a simplified example, as soon as TestInterface is more complicated, this doesn't really achieve its intended result.

interface TestInterface {
  movX: number,
  movY: number,
  anExtraKey: string
}

const key = `mov${str}` as keyof TestInterface; // is now "movX" | "movY" | "anExtraKey"

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.