Choose a version:
52% The original file has 154939 bytes (151.3k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 80629 bytes (78.7k, 52%).

After GZIP compression these minified files vary in size:
Boot
  24983 bytes (24.4k)
 CDN
cdnjs
  20899 bytes (20.4k)
 CDN
gzip -6 (default)
  20709 bytes (20.2k)
 local copy
gzip -9
  20632 bytes (20.1k)
 local copy
libdeflate -12
  19924 bytes (19.5k)
 local copy
7zip -mx=9 -tgzip
  19901 bytes (19.4k)
 local copy
zultra
  19881 bytes (19.4k)
 local copy
pigz -11 -n
  19862 bytes (19.4k)
 local copy
kzip -s0 -rn -b0
  19845 bytes (19.4k)
 local copy
Zopfli
  19831 bytes (19.4k)
 local copy
Zopfli (defluff)
  19830 bytes (19.4k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-1.0.5.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 1.0.5 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 --mls64 --bsr22 --lazy --ohh

(found July 1, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 64  --mls64
block splitting recursion 22  --bsr22
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/1.0.5/socket.io.min.js --location | md5sum
a978a909f01e282037513e942c32440e  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.0.5.min.zopfli.js.gz | md5sum
a978a909f01e282037513e942c32440e  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/1.0.5/socket.io.min.js --location | sha1sum
f31e760a00e18187569d60838b802cb4651bd3fa  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-1.0.5.min.zopfli.js.gz | sha1sum
f31e760a00e18187569d60838b802cb4651bd3fa  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 24983 bytes a978a909f01e282037513e942c32440e (invalid)
cdnjs 20899 bytes a978a909f01e282037513e942c32440e (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
19831 bytes -1 byte zopfli --i100000 --mls64 --bsr22 --lazy --ohh July 1, 2016 @ 16:55
19832 bytes -1 byte zopfli --i10000 --mls32 --bsr10 --lazy --ohh June 30, 2016 @ 09:46
19833 bytes -3 bytes zopfli --i1000 --mls16 --bsr13 --lazy --ohh June 29, 2016 @ 15:39
19836 bytes – zopfli --i100 --mls128 --bsr22 --lazy --ohh June 29, 2016 @ 12:20

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 @ 12:53.

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 19843 19843 19834 19833 19842 19841 19834 19834 19834 19848 19857 19849 19847 19847 19850
4 19839 19836 19842 19841 19837 19839 19855 19838 19854 19852 19853 19848 19848 19848 19852
6 19842 19838 19838 19837 19836 19838 19839 19836 19844 19852 19852 19847 19847 19853 19850
7 19836 19839 19837 19836 19835 19841 19839 19831 19831 19848 19848 19849 19848 19847 19852
8 19835 19832 19831 19836 19837 19838 19839 19834 19832 19847 19847 19847 19847 19847 19847
9 19836 19835 19836 19834 19834 19834 19834 19831 19833 19848 19847 19847 19847 19847 19848
10 19835 19835 19837 19833 19832 19833 19841 19833 19833 19847 19847 19847 19847 19847 19850
11 19836 19838 19838 19839 19836 19834 19839 19841 19833 19831 19847 19847 19847 19847 19848
12 19836 19837 19839 19836 19842 19838 19838 19846 19841 19856 19855 19847 19847 19848 19847
13 19836 19837 19839 19833 19833 19833 19834 19834 19834 19851 19847 19847 19847 19848 19848
14 19835 19834 19839 19839 19833 19833 19838 19833 19833 19848 19847 19847 19847 19848 19848
15 19836 19837 19838 19844 19836 19836 19839 19833 19833 19847 19847 19847 19847 19848 19848
16 19837 19837 19837 19835 19835 19842 19840 19839 19854 19831 19847 19847 19847 19847 19848
17 19835 19834 19835 19836 19838 19839 19839 19839 19833 19847 19847 19847 19847 19847 19850
18 19835 19835 19835 19834 19834 19836 19852 19833 19833 19851 19851 19851 19847 19849 19848
19 19835 19837 19838 19838 19834 19837 19845 19834 19834 19847 19847 19848 19847 19847 19847
20 19833 19833 19837 19836 19833 19840 19844 19833 19834 19848 19847 19847 19847 19848 19848
21 19836 19838 19839 19836 19836 19841 19840 19837 19833 19848 19847 19847 19847 19848 19848
22 19835 19834 19834 19831 19833 19831 19833 19835 19835 19848 19847 19847 19847 19848 19847
23 19842 19837 19837 19840 19836 19836 19839 19833 19833 19847 19847 19847 19847 19848 19847
25 19837 19837 19837 19836 19836 19838 19844 19836 19833 19853 19836 19847 19847 19847 19848
30 19836 19835 19833 19833 19831 19833 19838 19834 19833 19848 19847 19847 19847 19848 19848
40 19835 19835 19838 19833 19836 19831 19834 19834 19833 19847 19847 19847 19848 19848 19848

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 19836 bytes – 100%
1,000 19833 bytes -3 bytes 100%
10,000 19832 bytes -1 byte 100%
100,000 19831 bytes -1 byte 3.77%
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) 19845 bytes +14 bytes (+0.07%) –
1 19872 bytes +41 bytes (+0.21%) +27 bytes
2 19851 bytes +20 bytes (+0.10%) +6 bytes
3 19895 bytes +64 bytes (+0.32%) +50 bytes
4 19922 bytes +91 bytes (+0.46%) +77 bytes
5 19963 bytes +132 bytes (+0.67%) +118 bytes
6 19967 bytes +136 bytes (+0.69%) +122 bytes
7 20000 bytes +169 bytes (+0.85%) +155 bytes
8 20029 bytes +198 bytes (+1.00%) +184 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 16215 bytes -3616 bytes (-18.23%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 16946 bytes -2885 bytes (-14.55%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 17588 bytes -2243 bytes (-11.31%)
Brotli (Wikipedia) brotli brotli -q 11 18763 bytes -1068 bytes (-5.39%)
LZMA2 (Wikipedia) xz xz -9 19432 bytes -399 bytes (-2.01%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 19511 bytes -320 bytes (-1.61%)
Zstandard (Wikipedia) zstd zstd -19 19719 bytes -112 bytes (-0.56%)

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.