fetch - Missing boundary in multipart/form-data POST

Question

thanks for stopping by.

I want to send a new FormData() as the body of a POST request using the fetch api

the operation looks something like this

var formData = new FormData()  formData.append('myfile', file, 'someFileName.csv')    fetch('https://api.myapp.com',     {      method: 'POST',      headers: {        "Content-Type": "multipart/form-data"      },      body: formData    }  )

the problem here is that the boundary, something like

boundary=----WebKitFormBoundaryyEmKNDsBKjB7QEqu

never makes it into the Content-Type: header

it should look like this

Content-Type:multipart/form-data; boundary=----WebKitFormBoundaryyEmKNDsBKjB7QEqu

when you try the "same" operation with a new XMLHttpRequest(), like so

var request = new XMLHttpRequest()  request.open("POST", "https://api.mything.com")  request.withCredentials = true  request.send(formData)

the headers are correctly set

Content-Type:multipart/form-data; boundary=----WebKitFormBoundaryyEmKNDsBKjB7QEqu

so my question is,

1. how do I make fetch behave exactly like XMLHttpRequest in this situation?

2. if this is not possible, why?

Thanks everybody! This community is more or less the reason I have professional success.

Answer 1

The solution to the problem is to explicitly set Content-Type to undefined so that your browser or whatever client you're using can set it and add that boundary value in there for you. Disappointing but true.