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Uploading chunks of a large file using XHR2

I was having a conversation with my college friend about the enhancements in XHR2 and I was showing him how one can upload a file to the server using plain Ajax. He asked me whether I can upload a large file to the server such as a video file. I tried doing that but was bumped when the server reported with 400 error telling me that the 'POST size has exceeded maximum limit'. This got me thinking whether I can upload chunks of a large file to the server. I referred to the FileSystem API and came across the slice method that allows you to get a fragment of a file.

The slice method takes two parameters which specifies the start and end byte position of the fragment that is to be extracted. This fragment can then be uploaded to the server using XHR2.

Here’s the client code:

Here I’ve specified the chunk size as 1 MB (1024*1024) and then slicing the file such that each fragment is of 1 MB in size. Each of this fragments is then pushed to an array and then uploaded to the server using XHR2. Each fragment is a ByteArray and is sent to server synchronously.

Here’s the server side code:

The ByteArray is then written\appended to the file by calling the write method on the FileOutputStream object.


           Uploading chunk of size 1048576.
           Chunk of size 1048576 uploaded successfully.
           Uploading chunk of size 1048576.
           Chunk of size 1048576 uploaded successfully.
           Uploading chunk of size 1048576.
           Chunk of size 1048576 uploaded successfully.
           Uploading chunk of size 17540.
           Chunk of size 17540 uploaded successfully.

Here you can see that each fragment is of size 1 MB and the last fragment is of 17 KB. I've tried uploading large images and videos of size greater than 50 MB and it works fine.


  1. It is possible to send the file over Internet without uploading them anywhere by using a tool called Binfer. Binfer has no limitation on file size and quantity. You can send large files without any worry. Visit for details

  2. Hi, thank you for this useful script. (y)
    Won't it be better to send http headers(content-type & filename) only once ? or it does change with each sent chunk ???

  3. Another thing, this will cause some server-side problems if we try to use it on asynchronous sending mode, maybe if you replace the "while(start<size)" with a callback there won't be any server-side conflicts since the chunks will be sent one by one even when using async mode.

  4. Another thing, this will cause some server-side problems if we try to use it on asynchronous sending mode, maybe if you replace the while(start<size) with a callback on the upluadChunk function there wont be any server side conflict since the chunks will be sent one by one even while using async mode.
    (sorry for the double post)

  5. Duuude ! WTF ?!!!!!!
    mail me O.o

  6. oww my bad, it's the size tag lol

  7. The server should be able to get the data in correct order. If async is used then the blobs would not reach the server in correct order. This is the reason why the third field is set to 'false' when sending the XHR request.

  8. I think it is necessary to send the filename with each request. Once the first chunk of the file is uploaded it closes the stream and the session ends there. If there is a session maintained on the server then the it can store the filename in the session and the same may not be required to be sent in each request.

  9. yes friend i get that, i was trying to say that maybe if you use a callback on the uploadChunk function instead of calling it using WHILE, there won't be any chunks' order conflict when using async mode :-D

  10. it seems that i'am right,i just tested it :D
    you don't need to fix headers each time.

  11. sorry for double posting :-/
    my tests were a little wrong, the headers get overwritten by default values.
    but there is a way to avoid sending it each time.
    this can be handled on the server side by memorizing the first request's http values and then using them when all the chunks are received.

  12. Hi,

    Thanks for this post- can we also upload other fields along with this large file, in that case how it works should we send the data twice to server? once with only form-fields and and next large file-chunks.. please let me know if this is best approach to achieve, ?


  13. Chrome now forces you to use async=true with any XMLHttpRequest.


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