Symbol Keys

Every registration key can be a string or a symbol. Strings are convenient for app-wide public services. Symbols are useful when identity matters.

const DB = Symbol('db')
const CACHE = Symbol('cache')

const c = new Container()
  .registerValue('config', { dsn: 'postgres://localhost/app' })
  .registerClass(DB, PgPool, ['config'])
  .registerClass(CACHE, RedisPool, [])
  .registerClass('repo', UserRepo, [DB, CACHE])

c.get(DB)
c.get(CACHE)
c.get('repo')

When To Use Symbols

Pattern	Token
Private module-local service	Symbol('name')
Shared identity without imports	Symbol.for('name')
Nominal type-level distinction	unique symbol constant

Use local symbols for collectable private services. Symbol.for(name) is stored in the global symbol registry and is never garbage-collected.

Lazy Companions

The lazy companion key can also be a symbol:

const DB = Symbol('db')
const DB_LAZY = Symbol('dbLazy')

const c = new Container()
  .registerClass(DB, PgPool, [], 'singleton', DB_LAZY)

c.get(DB_LAZY).get()

The primary key and companion key do not need to be the same kind.