Choose a version:
52% The original file has 151203 bytes (147.7k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 78470 bytes (76.6k, 52%).

After GZIP compression these minified files vary in size:
Boot
  24715 bytes (24.1k)
 CDN
cdnjs
  20674 bytes (20.2k)
 CDN
gzip -6 (default)
  20512 bytes (20.0k)
 local copy
gzip -9
  20437 bytes (20.0k)
 local copy
libdeflate -12
  19756 bytes (19.3k)
 local copy
7zip -mx=9 -tgzip
  19712 bytes (19.3k)
 local copy
zultra
  19693 bytes (19.2k)
 local copy
pigz -11 -n
  19683 bytes (19.2k)
 local copy
kzip -s0 -rn -b2
  19671 bytes (19.2k)
 local copy
Zopfli
  19648 bytes (19.2k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.0.1.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 1.0.1 file, ETag caching is available and
if your browser doesn't support GZIP decompression then the uncompressed version will be sent.

Currently best Zopfli settings

You can use my super-compressed files for whatever purpose you like as long as you respect the library's original license agreement.
There are no restrictions from my side - but please avoid hot-linking if you run a high-traffic website.

These command-line settings yielded the best compression ratio so far (Linux version of zopfli-krzymod):
zopfli --i100000 --mb8 --mls128 --bsr15 --lazy --ohh

(found July 1, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 128  --mls128
block splitting recursion 15  --bsr15
lazy matching in LZ77 yes  --lazy
optimized Huffman headers yes  --ohh
initial random W for iterations 1  --rw1
initial random Z for iterations 2  --rz2

Verify file integrity

After decompression, my uncompressed files are identical to the original ones:

MD5:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.0.1/socket.io.min.js --location | md5sum
ba27831af5d11750796d29641295dd75  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.0.1.min.zopfli.js.gz | md5sum
ba27831af5d11750796d29641295dd75  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.0.1/socket.io.min.js --location | sha1sum
ed4365e569ab87c7f82effee7e9ef7c35dbc2965  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.0.1.min.zopfli.js.gz | sha1sum
ed4365e569ab87c7f82effee7e9ef7c35dbc2965  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 24715 bytes ba27831af5d11750796d29641295dd75 (invalid)
cdnjs 20674 bytes ba27831af5d11750796d29641295dd75 (invalid)

Note: only the MD5 hashes are shown to keep things simple.

Other Versions

Available Socket.IO versions at minime.stephan-brumme.com:

4.5.4, 4.5.3, 4.5.2, 4.5.1, 4.5.0, 4.4.1, 4.4.0, 4.3.2, 4.3.1, 4.3.0, 4.2.0, 4.1.3, 4.1.2, 4.1.1, 4.1.0, 4.0.2, 4.0.1, 4.0.0,
3.1.2, 3.1.1, 3.1.0, 3.0.5, 3.0.4, 3.0.3, 3.0.2, 3.0.1, 3.0.0,
2.4.0, 2.3.1, 2.3.0, 2.2.0, 2.1.1, 2.1.0, 2.0.4, 2.0.3, 2.0.2, 2.0.1, 2.0.0,
1.7.4, 1.7.2, 1.7.1, 1.7.0, 1.6.0, 1.5.0, 1.4.5, 1.4.4, 1.4.3, 1.4.2, 1.4.1, 1.4.0, 1.3.7, 1.3.6, 1.3.5, 1.3.4, 1.3.3, 1.3.2, 1.3.1, 1.3.0, 1.2.1, 1.2.0, 1.1.0, 1.0.6, 1.0.5, 1.0.4, 1.0.3, 1.0.2, 1.0.1, 1.0.0

The project site contains an overview how well these versions were compressed.
Other interesting projects are AngularJS, BackboneJS, Bootstrap, D3, Dojo, Ember, jQuery, Knockout, lodash, React, ThreeJS, UnderscoreJS and Vue.

Changelog

Best Zopfli parameters so far:
Size Improvement Parameters Found
19648 bytes -1 byte zopfli --i100000 --mls128 --bsr15 --lazy --ohh July 1, 2016 @ 13:40
19649 bytes -2 bytes zopfli --i100000 --mls64 --bsr15 --lazy --ohh June 29, 2016 @ 16:16
19651 bytes -2 bytes zopfli --i10000 --mls64 --bsr15 --lazy --ohh June 29, 2016 @ 15:36
19653 bytes -4 bytes zopfli --i1000 --mls64 --bsr15 --lazy --ohh June 29, 2016 @ 15:23
19657 bytes – zopfli --i100 --mls64 --bsr15 --lazy --ohh June 29, 2016 @ 11:23

If there are multiple parameter sets yielding the same compressed size, only the first one found is shown.

Most recent activity on July 20, 2020 @ 12:52.

Heatmaps

This Zopfli heatmap visualizes how compression changes when modifying the --bsr and --mls parameter.
Cell's contents is the best filesize achieved (in bytes, hover with mouse over cells to see number of iterations).

Good parameters are green, bad are red. The best and worst are bold as well.
The brightness of the blue background color indicates how many iterations were processed:
10,000 or 100,000.
bsr \ mls
2 4 8 16 32 64 128 256 512 1024 2048 4096 8192 16384 32768
bsr \ mls
2 4 8 16 32 64 128 256 512 1024 2048 4096 8192 16384 32768
2 19660 19660 19660 19660 19660 19661 19671 19661 19671 19672 19673 19673 19672 19667 19667
4 19660 19659 19660 19660 19661 19661 19675 19674 19675 19675 19673 19670 19681 19666 19667
6 19662 19662 19663 19662 19661 19659 19661 19676 19666 19674 19672 19666 19667 19666 19667
7 19662 19660 19661 19665 19663 19665 19662 19661 19672 19666 19673 19669 19667 19667 19669
8 19661 19660 19661 19658 19662 19660 19675 19661 19673 19673 19674 19673 19667 19666 19654
9 19661 19662 19666 19665 19672 19674 19673 19674 19672 19672 19682 19674 19666 19666 19653
10 19661 19661 19666 19665 19662 19666 19673 19659 19667 19667 19668 19667 19669 19666 19652
11 19661 19660 19661 19660 19661 19674 19659 19677 19674 19673 19673 19667 19666 19666 19666
12 19661 19660 19662 19660 19659 19659 19675 19676 19674 19674 19674 19674 19667 19671 19652
13 19661 19660 19661 19660 19659 19659 19675 19659 19673 19672 19675 19667 19658 19666 19661
14 19661 19665 19661 19659 19664 19659 19673 19676 19667 19672 19673 19666 19666 19666 19652
15 19661 19662 19661 19665 19661 19649 19648 19676 19667 19669 19666 19666 19666 19669 19669
16 19661 19660 19661 19665 19661 19659 19675 19661 19672 19672 19673 19676 19666 19666 19652
17 19661 19660 19662 19658 19665 19660 19676 19677 19674 19673 19675 19670 19669 19669 19673
18 19661 19661 19662 19660 19662 19659 19660 19674 19674 19674 19674 19672 19671 19671 19653
19 19662 19661 19661 19660 19659 19659 19657 19676 19666 19674 19672 19671 19667 19669 19652
20 19662 19663 19662 19660 19664 19661 19660 19661 19673 19674 19673 19667 19666 19669 19652
21 19662 19660 19660 19659 19665 19658 19658 19676 19673 19674 19673 19674 19669 19666 19654
22 19662 19664 19661 19659 19662 19659 19674 19676 19674 19672 19673 19667 19666 19669 19653
23 19661 19660 19662 19660 19668 19651 19651 19676 19667 19674 19674 19666 19667 19666 19652
25 19661 19661 19662 19659 19661 19662 19660 19676 19673 19674 19673 19673 19669 19666 19653
30 19661 19660 19661 19658 19664 19660 19675 19661 19673 19672 19674 19673 19666 19666 19652
40 19662 19660 19662 19661 19658 19659 19675 19676 19674 19674 19674 19673 19666 19666 19666

Due to the Monte Carlo design of my search algorithm, not all parameters have reached the same number of iterations yet:
Iterations Min. Bytes Reduction Coverage
100 19657 bytes – 100%
1,000 19653 bytes -4 bytes 100%
10,000 19650 bytes -3 bytes 100%
100,000 19648 bytes -2 bytes 0.58%
1,000,000 – – –
10,000,000 – – –

KZIP has far less options available for tuning/optimization. I only played around with the number of blocks (parameter -n):
Blocks Min. Bytes Compared To Best Zopfli Compared To Best KZIP
(any) 19672 bytes +24 bytes (+0.12%) +1 byte
1 19694 bytes +46 bytes (+0.23%) +23 bytes
2 19671 bytes +23 bytes (+0.12%) –
3 19719 bytes +71 bytes (+0.36%) +48 bytes
4 19741 bytes +93 bytes (+0.47%) +70 bytes
5 19776 bytes +128 bytes (+0.65%) +105 bytes
6 19807 bytes +159 bytes (+0.81%) +136 bytes
7 19816 bytes +168 bytes (+0.86%) +145 bytes
8 19851 bytes +203 bytes (+1.03%) +180 bytes

Non-DEFLATE Algorithms

Archivers based on completely different compression algorithms often produce superior results.
Unfortunately, browsers only support gzip compression at the moment.
However, support for Brotli is constantly growing - for example, your browser actually supports it !
Algorithm Program Parameters Size Compared To Best Zopfli
ZPAQ (Wikipedia) zpaq zpaq -method 69 16079 bytes -3569 bytes (-18.16%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 16798 bytes -2850 bytes (-14.51%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 17539 bytes -2109 bytes (-10.73%)
Brotli (Wikipedia) brotli brotli -q 11 18593 bytes -1055 bytes (-5.37%)
LZMA2 (Wikipedia) xz xz -9 19272 bytes -376 bytes (-1.91%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 19328 bytes -320 bytes (-1.63%)
Zstandard (Wikipedia) zstd zstd -19 19548 bytes -100 bytes (-0.51%)

Detailled Analysis

I wrote a DEFLATE decoder in Javascript. Click the button below to start a client-side analysis of the smallest gzipped files (may take a second):
Load and Parse Data


Notes: pigz is a fast open source multi-threaded implementation of gzip written by one of the original authors of gzip.
However, when using compression level 11, pigz actually switches to the slower Zopfli algorithm and isn't multi-threaded anymore.
KrzyMOD's extensions to Zopfli offer the highest level of configuration and is therefore used for my brute-force search.
Ken Silverman wrote the closed-source KZIP compression program and Jonathon Fowler ported it to Linux.
Defluff was created by Joachim Henke; DeflOpt is a tool by Ben Jos Walbeehm.

website made by Stephan Brumme in 2015 and still improving in 2026.
all timestamps are displayed in central european time. see my changelog.
no flash, not even images or external css files - and everything squeezed into a single html file.
which was handsomely compressed before releasing it into the wild internet - obviously.

please visit my homepage and my blog, too.
email: minime (at) stephan-brumme.com

All trademarks are property of their respective owners. You know, the boring legal stuff.