You have this error:

zlib.error: Error -3 while decompressing: incorrect header check

Which is most likely because you are trying to check headers that are not there, e.g. your data follows RFC 1951 (deflate compressed format) rather than RFC 1950 (zlib compressed format) or RFC 1952 (gzip compressed format).

choosing windowBits

But zlib can decompress all those formats:

• to (de-)compress deflate format, use wbits = -zlib.MAX_WBITS
• to (de-)compress zlib format, use wbits = zlib.MAX_WBITS
• to (de-)compress gzip format, use wbits = zlib.MAX_WBITS | 16

See documentation in http://www.zlib.net/manual.html#Advanced (section inflateInit2)

examples

test data:

>>> deflate_compress = zlib.compressobj(9, zlib.DEFLATED, -zlib.MAX_WBITS)
>>> zlib_compress = zlib.compressobj(9, zlib.DEFLATED, zlib.MAX_WBITS)
>>> gzip_compress = zlib.compressobj(9, zlib.DEFLATED, zlib.MAX_WBITS | 16)
>>> 
>>> text = '''test'''
>>> deflate_data = deflate_compress.compress(text) + deflate_compress.flush()
>>> zlib_data = zlib_compress.compress(text) + zlib_compress.flush()
>>> gzip_data = gzip_compress.compress(text) + gzip_compress.flush()
>>> 

obvious test for zlib:

>>> zlib.decompress(zlib_data)
'test'

test for deflate:

>>> zlib.decompress(deflate_data)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
zlib.error: Error -3 while decompressing data: incorrect header check
>>> zlib.decompress(deflate_data, -zlib.MAX_WBITS)
'test'

test for gzip:

>>> zlib.decompress(gzip_data)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
zlib.error: Error -3 while decompressing data: incorrect header check
>>> zlib.decompress(gzip_data, zlib.MAX_WBITS|16)
'test'

the data is also compatible with gzip module:

>>> import gzip
>>> import StringIO
>>> fio = StringIO.StringIO(gzip_data)  # io.BytesIO for Python 3
>>> f = gzip.GzipFile(fileobj=fio)
>>> f.read()
'test'
>>> f.close()

automatic header detection (zlib or gzip)

adding 32 to windowBits will trigger header detection

>>> zlib.decompress(gzip_data, zlib.MAX_WBITS|32)
'test'
>>> zlib.decompress(zlib_data, zlib.MAX_WBITS|32)
'test'

using gzip instead

or you can ignore zlib and use gzip module directly; but please remember that under the hood, gzip uses zlib.

fh = gzip.open('abc.gz', 'rb')
cdata = fh.read()
fh.close()

This looks like a bug:

readStream.avail_out = 0;

It should probably read:

readStream.avail_out = buffSize;

response.on('data', ...) can accept a Buffer, not just plain strings. When concatenating you are converting to string incorrectly, and then later can't gunzip. You have 2 options:

1) Collect all the buffers in an array, and in the end event concatentate them using Buffer.concat(). Then call gunzip on the result.

2) Use .pipe() and pipe the response to a gunzip object, piping the output of that to either a file stream or a string/buffer string if you want the result in memory.

Both options (1) and (2) are discussed here: http://nickfishman.com/post/49533681471/nodejs-http-requests-with-gzip-deflate-compression

The file is gzipped, so you need to use zlib.Gunzip instead of zlib.Inflate.

Also, streams are very efficient in terms of memory usage, so if you want to perform the retrieval without storing the .gz file locally first, you can use something like this:

request('https://wiki.mozilla.org/images/f/ff/Example.json.gz')
  .pipe(zlib.createGunzip())
  .pipe(fs.createWriteStream('example.json'));

Otherwise, you can modify your existing code:

var gunzip = zlib.createGunzip();
readstream.pipe(gunzip).pipe(writestream);

I hadn't realized that setting the UTF-8 encoding begins parsing in a different way. Removing the {encoding: 'UTF-8'} lets the zlib step decompress correctly, and my next step can consume directly from the stream.