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

August 2, 2023

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?

>Solution :

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]);
  }
}