Vercel Edge Function for an OpenAI API request

chat.ts

import type { NextRequest } from 'next/server'
import { createParser } from 'eventsource-parser'

export const config = {
  runtime: 'edge',
}

export default async function handler(req: NextRequest) {
  const encoder = new TextEncoder()
  const decoder = new TextDecoder()

  const readableStream = new ReadableStream({
    async start(controller) {
      const parser = createParser((event) => {
        if (event.type === 'event') {
          if (event.data === '[DONE]') {
            controller.close()
          } else {
            const data = JSON.parse(event.data)
            controller.enqueue(encoder.encode(data.choices[0].delta.content))
          }
        }
      })

      const response = await fetch('https://api.openai.com/v1/chat/completions', {
        method: 'POST',
        headers: {
          'Content-Type': 'application/json',
          Authorization: `Bearer ${process.env.OPENAI_API_KEY}`,
        },
        body: JSON.stringify({
          model: 'gpt-4',
          messages: [
            { role: 'system', content: system },
            { role: 'user', content: user },
          ],
          stream: true,
          temperature: 0,
        }),
      })

      if (!response.ok) {
        console.log(await response.text())
        throw new Error()
      }

      for await (const chunk of response.body as any) {
        parser.feed(decoder.decode(chunk))
      }
    },
  })

  return new Response(readableStream, {
    headers: { 'Content-Type': 'text/plain; charset=UTF-8' },
  })
}