Read file stream using JavaScript in web browser

Question

In web browser, I want to compute sha1 checksum of a huge file in the local filesystem without sending it to a server.

File API supports to read files from local disk but I guess it reads the entire of the file and put all of them into the memory. It may occur a problem if the file is larger than system memory.

Streams API seems to be useful to solve this problem but I couldn't find how to read a file using the API.

Is there any way to read file stream from local disk using javascript in web browser?

hmm. XMLHttpRequest seems to be able to open a blob url from URL.createObjectURL. — npcode
– npcode, Commented Sep 3, 2013 at 8:44
Did you ever find a solution to this problem? I'd like to know the same. — user1832464
– user1832464, Commented Dec 12, 2013 at 16:06
@monkeymatrix Not yet. I think the APIs should be improved to solve this problem. — npcode
– npcode, Commented Dec 14, 2013 at 13:07

Mat Gessel · Accepted Answer · 2017-01-26 01:32:05Z

13

The file api provides a slice method, so you should be able to read chunks of data

var blob = file.slice(startingByte, endindByte);

The Sha1 class in google's crypto api provides a update method, you should be able to feed the update method with your chunks

source:

edited Jan 26, 2017 at 1:32

Mat Gessel

6129 silver badges17 bronze badges

answered Apr 15, 2014 at 9:54

flotto

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5 Comments

npcode Over a year ago

Thanks a lot! It works very well and I write an example here: gist.github.com/npcode/11282867

BenKoshy Over a year ago

@npcode Thanks for the link - but the link is now dead?

npcode Over a year ago

@BKSpurgeon Yeah, you're right. Here is the new link: gist.github.com/eungjun-yi/11282867

Phil Over a year ago

Great that it works, but you chunk the file manually with a loop. Isn't the whole point of Streams that you don't have to do this annoying chunking yourself?

flotto Over a year ago

@Phil You're right, it would be better to you FileReader.onload, it already reads chunks.

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