created proxy for parser backend

src/app/api/proxy/paths.ts

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+export const PROXY_PATHS = {
+    saveParserBackend: '/api/proxy/saveParserBackend',
+}

src/app/api/proxy/proxyFunctions.ts

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+import type { NextRequest } from 'next/server'
+import { NextResponse } from 'next/server'
+
+/// Resend request to backend.
+/// Body is being sent as stream.
+export async function redirectRequest(
+    req: NextRequest,
+    proxyUrl: string,
+    backendBaseUrl: string
+): Promise<NextResponse> {
+    const reqUrl = req.url // full URL e.g. http://localhost:3000/api/proxy?id=123&foo=bar
+
+    // Find position of proxyPath in reqUrl
+    const proxyIndex = reqUrl.indexOf(proxyUrl)
+    if (proxyIndex === -1) {
+        return new NextResponse('Invalid proxy path', { status: 400 })
+    }
+
+    // Replace '/api/proxy/parserBackend' with backend URL, preserve everything after
+    // e.g. '/api/proxy/extra/path?id=123' -> 'http://your-backend-server/extra/path?id=123'
+    const backendUrl = backendBaseUrl + reqUrl.substring(proxyIndex + proxyUrl.length)
+    console.log('redirecting', req.url, 'to', backendUrl)
+
+    const backendRes = await fetch(backendUrl, {
+        method: req.method,
+        headers: req.headers,
+        body: req.body,
+        // @ts-ignore
+        duplex: 'half', // some property required by nodejs but not defined in @types/node
+    })
+
+    return new NextResponse(backendRes.body, {
+        status: backendRes.status,
+        headers: {
+            'Access-Control-Allow-Origin': '*',
+            'Content-Type': backendRes.headers.get('content-type') || 'application/octet-stream',
+        },
+    })
+}
+
+/// Browsers send OPTIONS request sometimes to check webserver allowed headers and methods.
+/// My backend doesn't support such requests so use proxy to answer to it that all headers and methods are allowed
+export async function responseToOPTIONS(req: NextRequest): Promise<NextResponse> {
+    console.log('proxy responding to CORS OPTIONS', req.url)
+
+    return new NextResponse(null, {
+        status: 200,
+        headers: {
+            'Access-Control-Allow-Origin': '*',
+            'Access-Control-Allow-Methods': 'GET, POST, PUT, PATCH, DELETE, HEAD',
+            'Access-Control-Allow-Headers':
+                req.headers.get('access-control-request-headers') || 'Content-Type, Authorization',
+        },
+    })
+}

src/app/api/proxy/saveParserBackend/[[...path]]/route.ts

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+import type { NextRequest } from 'next/server'
+import { NextResponse } from 'next/server'
+import { redirectRequest, responseToOPTIONS as respondToOPTIONS } from '@/app/api/proxy/proxyFunctions'
+import { appConfig } from '@/app/lib/configLoader'
+import { PROXY_PATHS } from '../../paths'
+
+async function redirect(req: NextRequest): Promise<NextResponse> {
+    return await redirectRequest(req, PROXY_PATHS.saveParserBackend, appConfig.services.saveParserBackend.url)
+}
+
+export const GET = redirect
+export const POST = redirect
+export const PUT = redirect
+export const PATCH = redirect
+export const DELETE = redirect
+export const HEAD = redirect
+export const OPTIONS = respondToOPTIONS