refactor: load server settings from JSON configuration

server/config.ts

@@ -0,0 +1,33 @@
+import { readFileSync } from 'node:fs';
+
+export type ServerConfig = {
+  host: string;
+  port: number;
+  mongoUri: string;
+  mongoDatabase: string;
+  visitStore: 'mongo' | 'memory';
+  allowedOrigins: string[];
+};
+
+export function loadServerConfig(path: string | URL = new URL('../.config.json', import.meta.url)): ServerConfig {
+  let contents: string;
+  try { contents = readFileSync(path, 'utf8'); }
+  catch { throw new Error('Cannot read .config.json; copy .config.example.json to .config.json and configure the server.'); }
+  let value: unknown;
+  try { value = JSON.parse(contents); }
+  catch { throw new Error('Invalid JSON in .config.json'); }
+  if (!value || typeof value !== 'object' || Array.isArray(value)) throw new Error('Invalid .config.json: expected an object');
+  const config = value as Record<string, unknown>;
+  const nonempty = (value: unknown): value is string => typeof value === 'string' && value.trim().length > 0;
+  for (const field of ['host', 'mongoUri', 'mongoDatabase']) {
+    if (!nonempty(config[field])) throw new Error(`Invalid .config.json field: ${field}`);
+  }
+  if (typeof config.port !== 'number' || !Number.isInteger(config.port) || config.port < 0 || config.port > 65535) {
+    throw new Error('Invalid .config.json field: port');
+  }
+  if (config.visitStore !== 'mongo' && config.visitStore !== 'memory') throw new Error('Invalid .config.json field: visitStore');
+  if (!Array.isArray(config.allowedOrigins) || !config.allowedOrigins.length || !config.allowedOrigins.every(nonempty)) {
+    throw new Error('Invalid .config.json field: allowedOrigins');
+  }
+  return config as ServerConfig;
+}