1---
2title: Dependency-Based Parallelization
3impact: CRITICAL
4impactDescription: 2-10× improvement
5tags: async, parallelization, dependencies, better-all
6---
7 
8## Dependency-Based Parallelization
9 
10For operations with partial dependencies, use `better-all` to maximize parallelism. It automatically starts each task at the earliest possible moment.
11 
12**Incorrect (profile waits for config unnecessarily):**
13 
14```typescript
15const [user, config] = await Promise.all([
16  fetchUser(),
17  fetchConfig()
18])
19const profile = await fetchProfile(user.id)
20```
21 
22**Correct (config and profile run in parallel):**
23 
24```typescript
25import { all } from 'better-all'
26 
27const { user, config, profile } = await all({
28  async user() { return fetchUser() },
29  async config() { return fetchConfig() },
30  async profile() {
31    return fetchProfile((await this.$.user).id)
32  }
33})
34```
35 
36**Alternative without extra dependencies:**
37 
38We can also create all the promises first, and do `Promise.all()` at the end.
39 
40```typescript
41const userPromise = fetchUser()
42const profilePromise = userPromise.then(user => fetchProfile(user.id))
43 
44const [user, config, profile] = await Promise.all([
45  userPromise,
46  fetchConfig(),
47  profilePromise
48])
49```
50 
51Reference: [https://github.com/shuding/better-all](https://github.com/shuding/better-all)
52