Request deduplication

Use case

Prevent duplicate concurrent requests to the same endpoint. If a request is already in flight, return the same promise instead of making a new network call.

Interceptor recipe

import { createClient } from '@parcely/core'
import type { HttpResponse } from '@parcely/core'

const http = createClient({ baseURL: 'https://api.example.com' })

const inFlight = new Map<string, Promise<HttpResponse<unknown>>>()

http.interceptors.request.use((config) => {
  const key = `${config.method ?? 'GET'}:${config.url}`
  const existing = inFlight.get(key)
  if (existing) {
    // Abort this request; caller will get the in-flight promise
    const controller = new AbortController()
    controller.abort()
    return { ...config, signal: controller.signal }
  }
  return config
})

A simpler approach is to deduplicate at the service layer:

const inFlight = new Map<string, Promise<unknown>>()

async function deduped<T>(key: string, fn: () => Promise<T>): Promise<T> {
  const existing = inFlight.get(key)
  if (existing) return existing as Promise<T>

  const promise = fn().finally(() => inFlight.delete(key))
  inFlight.set(key, promise)
  return promise
}

// Usage:
const user = await deduped('user:me', () =>
  http.get('/users/me').then((r) => r.data),
)

Notes

• Deduplication is most useful for GET requests. Avoid deduplicating POST/PUT/DELETE.
• The in-flight map uses string keys. Customise the key generation for requests with different params.
• Clean up the map entry in .finally() to prevent memory leaks.