Late dependencies

dependencies.js

export default new Proxy(new Map(), {
  get: (target, name) => {
    if (!target.has(name)) {
      target.set(name, [[]])
    }

    const entry = target.get(name)
    const [resolvers, value] = entry

    return entry.length === 2
      ? Promise.resolve(value)
      : new Promise(resolve => resolvers.push(resolve))
  },
  set: (target, name, value) => {
    if (!target.has(name)) {
      target.set(name, [[]])
    }

    const entry = target.get(name) ?? [[]]
    const [resolvers] = entry

    if (entry.length === 2) {
      throw new Error(`Unable to redefine dependency: ${name}`)
    }

    entry.push(value)

    for (const resolve of resolvers) {
      resolve(value)
    }
  }
})