Typescript: Type Guard for environment variable as key of JSON object

Issue

I'm trying to write a Type Guard, such that TypeScript doesn't throw an error anymore on the last line where I try to load data based on a certain key. Somehow TypeScript still things that the environment variable is a string rather than a known key of the object. Since it now throws: No index signature with a parameter of type 'string' was found on type....

Am I missing some edge case here where the environment variable could still be undefined as a key?

import JsonData from '../data/data.json'

const doesKeyExist: (
  input: string | undefined
) => boolean = (input) =>
  input && JsonData.hasOwnProperty(input)

if (!doesKeyExist(process.env.SOME_VARIABLE))
  throw Error('Environment Variable not declared!')

const data = JsonData[process.env.NEXT_PUBLIC_TENANT_ID]

Solution

This seems to do the trick:

import JsonData from '../data/data.json'

function doesKeyExist(
  input: string | undefined
): input is keyof typeof CategoriesData {
  return !!(input && CategoriesData.hasOwnProperty(input))
}

if (!doesKeyExist(process.env.SOME_VARIABLE))
  throw Error('Environment Variable not declared, or key does not exist in config file!')

const data = JsonData[process.env.NEXT_PUBLIC_TENANT_ID]

Answered By - maus