qcast-front/.agents/skills/vercel-react-best-practices/rules/async-dependencies.md

title	impact	impactDescription	tags
Dependency-Based Parallelization	CRITICAL	2-10× improvement	async, parallelization, dependencies, better-all

Dependency-Based Parallelization

For operations with partial dependencies, use better-all to maximize parallelism. It automatically starts each task at the earliest possible moment.

Incorrect (profile waits for config unnecessarily):

const [user, config] = await Promise.all([
  fetchUser(),
  fetchConfig()
])
const profile = await fetchProfile(user.id)

Correct (config and profile run in parallel):

import { all } from 'better-all'

const { user, config, profile } = await all({
  async user() { return fetchUser() },
  async config() { return fetchConfig() },
  async profile() {
    return fetchProfile((await this.$.user).id)
  }
})

Alternative without extra dependencies:

We can also create all the promises first, and do Promise.all() at the end.

const userPromise = fetchUser()
const profilePromise = userPromise.then(user => fetchProfile(user.id))

const [user, config, profile] = await Promise.all([
  userPromise,
  fetchConfig(),
  profilePromise
])

Reference: https://github.com/shuding/better-all