feat: load config files from local fs and fallback if needed

config/loader.ts

@@ -0,0 +1,49 @@
+import type { JsonValue } from '@std/json'
+import defaultAccounts from './account.json' with { type: 'json' }
+import defaultDkim from './dkim.json' with { type: 'json' }
+
+function checkFsErrors(error: Error) {
+	if (error instanceof Deno.errors.NotFound) {
+		//use default config file
+		return
+	}
+	if (error instanceof Deno.errors.PermissionDenied) {
+		throw new Error(
+			'unable to load config file due to read access permissions issues',
+			{
+				cause: error,
+			},
+		)
+	}
+	throw error
+}
+
+type JsonRecord = Record<string, JsonValue>
+export type DkimRecord = {
+	domainName: string
+	keySelector: string
+	privateKey: string
+}
+export type AccountRecord = Record<string, {
+	name: string
+	address: string
+}>
+
+async function readJsonFile<
+	T extends JsonRecord,
+>(path: string, defaultValue: T): Promise<T> {
+	const file = await Deno.readTextFile(path).catch(checkFsErrors)
+	const json = JSON.parse(file ?? '{}')
+	return { ...defaultValue, ...json }
+}
+
+const basePath = '/ect/cohabit/mailer'
+
+export const accounts = await readJsonFile<AccountRecord>(
+	`${basePath}/account.json`,
+	defaultAccounts,
+)
+export const dkim = await readJsonFile<DkimRecord>(
+	`${basePath}/dkim.json`,
+	defaultDkim,
+)

src/contact.ts

@@ -1,5 +1,4 @@
-import accounts from '../config/account.json' with { type: 'json' }
+import { accounts, dkim } from '../config/loader.ts'
-import dkim from '../config/dkim.json' with { type: 'json' }
 
 export type AddressString = `${string}@${string}.${string}`
 
@@ -22,7 +21,7 @@ export class Contact {
 			throw new Error('unknown short name contact')
 		}
 
-		const { name, address } = accounts[shortName as keyof typeof accounts]
+		const { name, address } = accounts[shortName]
 
 		if (typeof name !== 'string') {
 			throw new SyntaxError(

src/transporter.ts

@@ -1,6 +1,6 @@
 // @deno-types="npm:@types/nodemailer@^6.4.15"
 import nodemailer from 'nodemailer'
-import dkim from '../config/dkim.json' with { type: 'json' }
+import { dkim } from '../config/loader.ts'
 
 export async function transporter() {
 	const dkimPath = dkim.privateKey