Choose a version:
37% The original file has 119021 bytes (116.2k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 44191 bytes (43.2k, 37%).

After GZIP compression these minified files vary in size:
gzip -6 (default)
  13133 bytes (12.8k)
 local copy
cdnjs
  13101 bytes (12.8k)
 CDN
gzip -9
  13074 bytes (12.8k)
 local copy
kzip -s0 -rn -b2
  12659 bytes (12.4k)
 local copy
libdeflate -12
  12639 bytes (12.3k)
 local copy
7zip -mx=9 -tgzip
  12631 bytes (12.3k)
 local copy
zultra
  12625 bytes (12.3k)
 local copy
pigz -11 -n
  12613 bytes (12.3k)
 local copy
Zopfli
  12599 bytes (12.3k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/socketio/socket.io-4.5.4.min.js (or via HTTPS)

You will automatically get the smallest Socket.IO 4.5.4 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 --i1000000 --mb8 --mls4096 --bsr4 --lazy --ohh

(found January 5, 2023)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 4096  --mls4096
block splitting recursion 4  --bsr4
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/4.5.4/socket.io.min.js --location | md5sum
db9bf2a88958a37857fb8f7b56e0fe04  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.5.4.min.zopfli.js.gz | md5sum
db9bf2a88958a37857fb8f7b56e0fe04  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/socketio/socket.io-client/4.5.4/socket.io.min.js --location | sha1sum
6bac03d130a224746a6ab405f519535088a6708e  -
curl --silent --compressed https://minime.stephan-brumme.com/files/socket.io/socket.io-4.5.4.min.zopfli.js.gz | sha1sum
6bac03d130a224746a6ab405f519535088a6708e  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
cdnjs 13101 bytes db9bf2a88958a37857fb8f7b56e0fe04 (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
12599 bytes -1 byte zopfli --i1000000 --mls4096 --bsr4 --lazy --ohh January 5, 2023 @ 15:37
12600 bytes -1 byte zopfli --i100000 --mls4096 --bsr4 --lazy --ohh January 4, 2023 @ 11:07
12601 bytes -1 byte zopfli --i10000 --mls4096 --bsr4 --lazy --ohh January 4, 2023 @ 10:52
12602 bytes -1 byte zopfli --i1000 --mls4096 --bsr4 --lazy --ohh January 4, 2023 @ 00:50
12603 bytes -1 byte zopfli --i1000 --mls4096 --bsr8 --lazy --ohh January 3, 2023 @ 21:18
12604 bytes -3 bytes zopfli --i1000 --mls8192 --bsr10 --lazy --ohh January 3, 2023 @ 18:17
12607 bytes – zopfli --i100 --mls8192 --bsr8 --lazy --ohh January 3, 2023 @ 16:37

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

Most recent activity on January 5, 2023 @ 15:37.

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 1,000,000.
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 12608 12609 12609 12608 12607 12609 12610 12606 12605 12611 12609 12609 12607 12610 12611
4 12607 12608 12609 12608 12607 12607 12610 12607 12608 12608 12605 12599 12609 12608 12608
6 12610 12609 12608 12607 12609 12608 12607 12606 12606 12612 12607 12608 12611 12610 12609
7 12607 12607 12606 12607 12611 12607 12605 12606 12609 12607 12606 12603 12603 12605 12607
8 12608 12608 12607 12609 12605 12608 12607 12606 12605 12611 12605 12603 12602 12608 12619
9 12609 12610 12609 12607 12606 12610 12610 12606 12608 12606 12605 12602 12611 12609 12608
10 12610 12610 12606 12605 12606 12611 12606 12606 12609 12609 12606 12610 12603 12607 12617
11 12613 12608 12606 12610 12611 12609 12609 12606 12609 12610 12606 12609 12603 12613 12612
12 12608 12606 12609 12607 12608 12607 12609 12606 12608 12606 12606 12602 12603 12605 12605
13 12605 12609 12609 12612 12608 12607 12609 12606 12605 12609 12605 12609 12612 12606 12610
14 12610 12610 12609 12607 12610 12611 12605 12610 12608 12610 12606 12603 12611 12614 12611
15 12614 12609 12609 12607 12609 12607 12608 12606 12607 12606 12606 12609 12603 12605 12607
16 12605 12606 12609 12609 12609 12609 12605 12606 12607 12610 12605 12602 12610 12615 12612
17 12608 12607 12606 12607 12607 12609 12605 12606 12609 12609 12605 12602 12609 12613 12606
18 12610 12607 12605 12606 12607 12609 12607 12606 12609 12605 12605 12603 12611 12609 12608
19 12612 12609 12611 12607 12609 12610 12605 12605 12607 12610 12605 12603 12603 12610 12609
20 12607 12608 12610 12608 12606 12609 12609 12610 12609 12609 12610 12609 12610 12606 12609
21 12608 12608 12609 12607 12606 12607 12605 12606 12609 12606 12609 12611 12603 12609 12609
22 12613 12610 12607 12608 12609 12610 12610 12606 12608 12611 12605 12604 12609 12605 12610
23 12609 12605 12606 12606 12608 12609 12610 12606 12606 12612 12610 12609 12603 12607 12607
25 12614 12610 12607 12609 12605 12608 12608 12606 12607 12612 12607 12606 12612 12605 12606
30 12608 12606 12606 12607 12607 12609 12607 12606 12607 12610 12605 12603 12610 12605 12613
40 12606 12610 12608 12608 12608 12606 12609 12606 12609 12610 12606 12602 12611 12605 12614

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 12607 bytes – 100%
1,000 12602 bytes -5 bytes 100%
10,000 12601 bytes -1 byte 100%
100,000 12600 bytes -1 byte 0.29%
1,000,000 12599 bytes -1 byte 0.29%
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) 12662 bytes +63 bytes (+0.50%) +3 bytes
1 12662 bytes +63 bytes (+0.50%) +3 bytes
2 12659 bytes +60 bytes (+0.48%) –
3 12701 bytes +102 bytes (+0.81%) +42 bytes
4 12692 bytes +93 bytes (+0.74%) +33 bytes
5 12716 bytes +117 bytes (+0.93%) +57 bytes
6 12750 bytes +151 bytes (+1.20%) +91 bytes
7 12768 bytes +169 bytes (+1.34%) +109 bytes
8 12801 bytes +202 bytes (+1.60%) +142 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
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 10729 bytes -1870 bytes (-14.84%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 11125 bytes -1474 bytes (-11.70%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 11678 bytes -921 bytes (-7.31%)
Brotli (Wikipedia) brotli brotli -q 11 11795 bytes -804 bytes (-6.38%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 12277 bytes -322 bytes (-2.56%)
LZMA2 (Wikipedia) xz xz -9 12520 bytes -79 bytes (-0.63%)
Zstandard (Wikipedia) zstd zstd -19 12560 bytes -39 bytes (-0.31%)

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.