getEnvVar and getEnvFlag type-safe functions

env.ts

type EnvVarName =
  | 'FOO'
  | 'BAR';

type EnvFlagName = 
  | 'IS_FOO'
  | 'IS_BAR';

export function getEnvVar<T extends string = EnvVarName | EnvFlagName>(
	name: T,
	params?: {
		default?: string;
	},
): string;
export function getEnvVar<T extends string = EnvVarName | EnvFlagName>(
	name: T,
	params?: {
		default?: string;
		optional: true;
	},
): string | undefined;
export function getEnvVar<T extends string = EnvVarName | EnvFlagName>(
	name: T,
	params?: {
		default?: string;
		optional?: true;
	},
): string | undefined {
	const res = process.env[name] ?? params?.default;
	if (typeof res === 'undefined') {
		if (params?.optional) {
			return undefined;
		}
		throw new Error(`Environment variable '${name}' is not defined`);
	}
	return res;
}

export function getEnvFlag<T extends string = EnvFlagName>(name: T): boolean {
	const str = getEnvVar(name, { default: '0' });
	return ['1', 'true'].includes(str);
}