19 Mar 2018

This article will take about 2 minutes to read.
No comments

Today I learned:

To upload files using fetch and FormData is supported in IE10+. you must not set Content-Type header.

const fileInput = document.querySelector('#your-file-input') ;
const formData = new FormData();

formData.append('file', fileInput.files[0]);

const options = {
  method: 'POST',
  body: formData,
  // If you add this, upload won't work
  // headers: {
  //   'Content-Type': 'multipart/form-data',
  // }
};

fetch('your-upload-url', options);

Problem I had

My API wrapper class has default content type header set to:
'Content-Type': 'application/json'

So I thought, to upload files using FormData, it would be enough to override it with:
'Content-Type': 'multipart/form-data'

But alas, it didn’t work, server couldn’t parse the files I was uploading. I’ve wasted about half an hour, and then noticed that simple HTML form was setting something else:

Content-Type: multipart/form-data; boundary=----WebKitFormBoundaryIn312MOjBWdkffIM

It had this boundary thing I didn’t know anything about.

Then I started searching around the internet and found the solution. To set the correct boundary, I had to explicitly delete Content-Type header. In that case browser will set the correct boundary itself.

Adding this line solved it.

// Remove 'Content-Type' header to allow browser to add
// along with the correct 'boundary'
delete options.headers['Content-Type'];

Explanation

What is boundary and why I had to delete the header?

Multipart form allow transfer of binary data, therefore server needs a way to know where one field’s data ends and where the next one starts.

That’s where boundary comes in. It defines a delimiter between fields we are sending in our request (similar to & for GET requests). You can define it yourself, but it is much easier to let browser do it for you.

Example payload:

------WebKitFormBoundaryIn312MOjBWdkffIM
Content-Disposition: form-data; name="file"; filename="my-image.jpg"
Content-Type: image/jpeg


------WebKitFormBoundaryIn312MOjBWdkffIM
Content-Disposition: form-data; name="field"

imagePortrait
------WebKitFormBoundaryIn312MOjBWdkffIM--

That’s why I had to manually delete existing header, as it didn’t contain boundary, and server was unable to parse the file correctly.

Read more about it on this StackOverflow question and MDN.

Category
JavaScript

No comments, be the first to comment

Sending failed, please try again.
Thank you! Your comment is sent. Please note that all of the comments go through moderation.