How do I send a acknowledgement to client, perform work in the background and then send the data back to the client?

Trying to create a rest api that processes excel data. I want to let the client know that the files have been received.

app.post('/upload',upload.fields([{name: 'raw'},{name: 'report'}]), (req: any, res: any)=>{
    
    res.send(statuscode 200)

    // do async excel operations

    return res.json({data: data});
})

I don't want to send a response after the client has been waiting for 30 seconds

await excelOperations();
return res.json()

With the traditional HTTP request-response model, you don't. You send the acknowledgement as the response to the data submission, when the upload has completed. Then the client makes another request (or even multiple ones) to inquire about the status of the processing, which you respond to with a progress report response each, and finally the client makes another request to download the results, to which you respond with the processed resource.

You could however use server-sent events or websockets which would allow you to send multiple things as long as the connection is open.

To complete Bergi answer you can do something like that :

• Create a joblist object
• When hitting the /upload endpoint, before uploading create a new job id with a status "pending" and return the job id as response
• When upload complete (after your async code) change the status of the job to completed
• Create a new endpoint [GET] /job/:jobId that return status of a job stored in your job object
• On the front side check in interval the endpoint /job/:jobId with the job id returned by [POST] / upload

> This code is just an example to explain the answer, adapt it to your need, handle error etc

Code example

const NanoidAsync = require('nanoid/async');
let jobList = {}

app.post('/upload',upload.fields([{name: 'raw'},{name: 'report'}]), (req: any, res: any)=>{
    
    let jobId = await NanoidAsync.nanoid();
    jobList[jobId] = {jobId:jobId,status:"pending"}
    res.send(jobList[jobId])
    await excelOperations();

})

app.get('/job/:jobId', (req: any, res: any)=>{
    res.send(jobList[jobId])
})

As asked in comment , what to do after ?

If your job doesn't need return it's just processing something just display on the front side when you detect job success a message or update button text or whatever yo uwant to tell the user (if needed) that job is completed

If you job must return data, edit the code as follow :

app.post('/upload',upload.fields([{name: 'raw'},{name: 'report'}]), (req: any, res: any)=>{

    let jobId = await NanoidAsync.nanoid();
    jobList[jobId] = {jobId:jobId,status:"pending",data:null} // New code
    res.send(jobList[jobId])
    jobList[jobId].data = await excelOperations(); // New code
    
})

And on the front side you will get as return of [GET] /job/:jobId the data returned by your job

Advice to put a code like that in production

• Manage a lifecycle of your jobs data, after X time , delete the key in your job list to avoid having a big object in memory
• Manage error