💎 Zod - `X` or `X_FILE` transformer

mod.ts

import { z } from 'zod';

function transformFileProperty<T extends string>(key: T) {
	return (data: Record<string, string>) => {
		if (data[key]) {
			return {
				[key]: data[key],
			} as {
				[P in T]: string;
			};
		}

		if (data[`${key}_FILE`]) {
			return {
				[key]: Deno.readTextFileSync(data[`${key}_FILE`]).trim(),
			} as {
				[P in T]: string;
			};
		}

		throw new Error(
			`Either \`${key}\` or \`${key}_FILE\` must be provided`,
		);
	};
}

const MySchema = z
	.object({
		MY_SECRET: z.string(),
		MY_SECRET_FILE: z.string(),
	})
	.partial()
	.transform(transformFileProperty('MY_SECRET'));