Business
Jobs
  • About Us
  • Solutions
    • Job Postings
      Post your job and receive qualified candidates in 48h.
    • Candidate Assessments
      500+ technical and psychological tests, plus anti-fraud.
    • Headhunting
      Tailor-made executive search from start to finish.
    • Payroll + EOR
      Payroll dispersal and EOR across 15+ LATAM countries.
  • Pricing
  • Jobs

0

290
Views
How can I upload file and also include JSON data with Fetch API?

Hope you're having a good day. So I'm using Fetch API to upload files to server but I also want to include a string as JSON for my backend. But I'm unable to do so.

Right now this is what my upload-file function looks like. This works perfectly if I want to just upload a file and not include a string as JSON:

const uploadFile = ( e ) => {

    const uploadedFile = e.target.files[ 0 ];
    const formData     = new FormData();

    formData.append( 'data', uploadedFile );

    const rawResponse = await fetch( '/upload-file', {
          method: 'POST',
          body: formData
    });
    
};

uploadFile();

Getting the file in my NodeJS backend with req.files.data;

But if I include a string as JSON and try to upload file then I get undefined in my NodeJS backend. This is how it looks like with string as JSON:

const uploadFile = ( e ) => {

    const uploadedFile = e.target.files[ 0 ];
    const formData     = new FormData();
    const str          = 'from client';

    formData.append( 'data', uploadedFile );

    const rawResponse = await fetch( '/upload-file', {
          method: 'POST',
          body: {
                file: formData,
                str 
          }
    });
    
};

uploadFile();

Now if I want to get the file in my backend with:

req.files.file, I get undefined.

req.body, I get { 'object Object': '' }.

req.files.data, I get UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'data' of undefined.

req.body.data, I get undefined.

What is going wrong here?

about 4 years ago · Juan Pablo Isaza
2 answers
Answer question

0

you append it to FormData, not body

const uploadFile = ( e ) => {

    const uploadedFile = e.target.files[ 0 ];
    const formData     = new FormData();
    const str          = 'from client';

    formData.append( 'data', uploadedFile );
    formData.append( 'str', str );

    const rawResponse = await fetch( '/upload-file', {
          method: 'POST',
          body: formData
    });
    
};

uploadFile();
about 4 years ago · Juan Pablo Isaza Report

0

Just append the value to the FormData object, same as you did with the file already

formData.append( 'str', '["some JSON here"]' ); 

And then keep sending just that object, body: formData

about 4 years ago · Juan Pablo Isaza Report
Answer question
Find remote jobs

Discover the new way to find a job!

Top jobs
Top job categories
Business
Post vacancy Pricing Sales
Legal
Terms and conditions Privacy policy
© 2026 PeakU Inc. All Rights Reserved.
Andres GPT
Show me some job opportunities
There's an error!