fetch - Missing boundary in multipart/form-data POST

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.

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.

I removed "Content-Type" and added 'Accept' to http headers and it worked for me. Here are the headers I used,

'headers': new HttpHeaders({
        // 'Content-Type': undefined,
        'Accept': '*/*',
        'Authorization': 
        "Bearer "+(JSON.parse(sessionStorage.getItem('token')).token),
        'Access-Control-Allow-Origin': this.apiURL,
        'Access-Control-Allow-Methods': 'GET, POST, OPTIONS, PUT, PATCH, DELETE',
        'Access-Control-Allow-Headers': 'origin,X-Requested-With,content-type,accept',
        'Access-Control-Allow-Credentials': 'true' 
       
      })

fetch(url,options)

1. If you set a string as options.body, you have to set the Content-Type in request header ,or it will be text/plain by default.
2. If options.body is specific object like let a = new FormData() or let b = new URLSearchParams(), you don't have to set the Content-Type by hand.It will be added automaticlly.

• for a ,it will be something like

  multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW

as you see, the boundary is automaticlly added.

• for b, it is application/x-www-form-urlencoded;

I had the same issue, and was able to fix it by excluding the Content-Type property, allowing the browser to detect and set the boundary and content type automatically.

Your code becomes:

var formData = new FormData()
formData.append('myfile', file, 'someFileName.csv')

fetch('https://api.myapp.com',
  {
    method: 'POST',
    body: formData
  }
)

Add headers:{content-type: undefined} browser will generate a boundary for you that is for uploading a file part-and-part with streaming if you are adding 'multiple/form-data' it means you should create streaming and upload your file part-and-part

So it is okay to add request.headers = {content-type: undefined}

I'm using the aurelia-api (an wrapper to aurelia-fetch-client). In this case the Content-Type default is 'application/json'. So I set the Content-Type to undefined and it worked like a charm.