Typescript: for .. in - define variable as keyof instead of string

Question

When I iterate over keys in TypeScript the variable is being assigned the type of string instead of type keyof object that I enumerate.

Example:

class Files {
  patches?: Data;
  website?: Data;
  github?: Data;
}

const files: Files = {};

for (const file in Files.prototype) {
  if (files[file]) {
    console.log(files[file])
  }
}

Error: Element implicitly has an 'any' type because expression of type 'string' can't be used to index type 'Files'. No index signature with a parameter of type 'string' was found on type 'Files'.

How can I solve this?

There are many Q&A on that error message. Please check. For instance this answer to use keyof type assertion. — trincot
– trincot, Commented Aug 2, 2023 at 17:22

wonderflame · Accepted Answer · 2023-08-02 17:16:12Z

2

You can declare the file outside of the for in with the correct type:

const files: Files = {};

let file: keyof typeof Files.prototype;

for (file in Files.prototype) {
  if (files[file]) {
    console.log(files[file]);
  }
}

answered Aug 2, 2023 at 17:16

wonderflame

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2 Comments

Maciej Bledkowski Over a year ago

Thank you very much for the answer. Is there any other option, if I want to define file variable inside the for parameters?

wonderflame Over a year ago

If you want to go without the assertion then I don't think there is any other way to do it with for in

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