Choose a version:
23% The original file has 178210 bytes (174.0k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 40939 bytes (40.0k, 23%).

After GZIP compression these minified files vary in size:
Microsoft
  18718 bytes (18.3k)
CDN
Boot
  17149 bytes (16.7k)
CDN
unpkg
  15194 bytes (14.8k)
CDN
cdnjs
  15105 bytes (14.8k)
CDN
gzip -6 (default)
  15035 bytes (14.7k)
local copy
cdnhttps
  15013 bytes (14.7k)
CDN
gzip -9
  15010 bytes (14.7k)
local copy
7zip -mx=9 -tgzip
  14587 bytes (14.2k)
local copy
libdeflate -12
  14577 bytes (14.2k)
local copy
kzip -s0 -rn -b2
  14542 bytes (14.2k)
local copy
pigz -11 -n
  14541 bytes (14.2k)
local copy
Zopfli
  14521 bytes (14.2k)
local copy
Zopfli (defluff)
  14520 bytes (14.2k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/knockout/knockout-2.2.1.min.js

You will automatically get the smallest Knockout 2.2.1 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

Save 492 bytes by using my Knockout 2.2.1 Zopfli version instead of the best available CDN (3.39% smaller than cdnhttps, 14521 vs. 15013 bytes):
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 --mls32 --bsr9 --lazy --ohh

(found December 18, 2015)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 32  --mls32
block splitting recursion 9  --bsr9
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

Even Smaller Files Thanks To Defluff

Zopfli's output can be further optimized by the defluff tool.
In this particular case, defluff saves 1 more byte (14520 bytes).

Verify file integrity

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

MD5:
curl --silent --compressed http://knockoutjs.com/downloads/knockout-2.2.1.js --location | md5sum
8764a417a2bc3ea8abd0e7fb7153bf00  -
curl --silent --compressed http://minime.stephan-brumme.com/files/knockout/knockout-2.2.1.min.zopfli.js.gz | md5sum
8764a417a2bc3ea8abd0e7fb7153bf00  -

SHA1:
curl --silent --compressed http://knockoutjs.com/downloads/knockout-2.2.1.js --location | sha1sum
23a818cc0126a80f730d2a49b0393594e8f68dbf  -
curl --silent --compressed http://minime.stephan-brumme.com/files/knockout/knockout-2.2.1.min.zopfli.js.gz | sha1sum
23a818cc0126a80f730d2a49b0393594e8f68dbf  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Microsoft 18718 bytes 8764a417a2bc3ea8abd0e7fb7153bf00 (invalid)
Boot 17149 bytes 8764a417a2bc3ea8abd0e7fb7153bf00 (invalid)
cdnjs 15105 bytes 8764a417a2bc3ea8abd0e7fb7153bf00 (invalid)
cdnhttps 15013 bytes 8764a417a2bc3ea8abd0e7fb7153bf00 December 24, 2015 @ 07:33

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
unpkg 15194 bytes decccaf1987d405cff2363d65de76f2e < (function() {function j(w){throw w;}var m=!0,p=null,r=!1;f [...]
< function L(w){function ha(a,d,c,e,f){var g=[];a=b.j(functi [...]
< h=Math.max,k=[],l,n=a.length,q,s=b.length,v=s-n||1,G=n+s+1 [...]
< 0;k=v[e];e++)if(t.value===k.value){t.moved=k.index;k.moved [...]
< default:j(Error("Unknown renderMode: "+d))}g&&(U(c,e),f.af [...]
< e[g];"function"===typeof h?(g=h(a[f].value))&&j(Error(g)): [...]
< d(n,a):d)||b.J.instance.getBindings(a,n)){if(0===h){h=1;fo [...]
< typeof v.update&&(0,v.update)(a,f(s),g,q,n)}},p,{W:a});ret [...]
< a.nodeValue));return p}function H(a){return 8==a.nodeType& [...]
< g;break;case "object":case "undefined":var h=c.get(g);f[e] [...]
[...]
July 11, 2016 @ 16:43

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

Other Versions

Available Knockout versions at minime.stephan-brumme.com:

3.4.2, 3.4.1, 3.4.0,
3.3.0,
3.2.0,
3.1.0,
3.0.0,
2.3.0,
2.2.1, 2.2.0,
2.1.0,
2.0.0,
1.2.1, 1.2.0,
1.1.2, 1.1.1, 1.1.0

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

Changelog

Best Zopfli parameters so far:
Size Improvement Parameters Found
14521 bytes -1 byte zopfli --i100000 --mls32 --bsr9 --lazy --ohh December 18, 2015 @ 13:35
14522 bytes -2 bytes zopfli --i10000 --mls32 --bsr9 --lazy --ohh September 19, 2015 @ 17:18
14524 bytes -1 byte zopfli --i1000 --mls32 --bsr9 --lazy --ohh September 18, 2015 @ 17:48
14525 bytes -1 byte zopfli --i10000 --mls8192 --bsr7 --lazy --ohh September 18, 2015 @ 05:51
14526 bytes -1 byte zopfli --i1000 --mls8192 --bsr14 --lazy --ohh September 10, 2015 @ 11:54
14527 bytes -2 bytes zopfli --i1000 --mls8192 --bsr8 --lazy --ohh September 10, 2015 @ 11:52
14529 bytes -2 bytes zopfli --i1000 --mls8192 --bsr2 --lazy --ohh September 10, 2015 @ 11:52
14531 bytes zopfli --i100 --mls8192 --bsr13 --lazy --ohh September 10, 2015 @ 11:36

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

Most recent activity on July 11, 2016 @ 16:43.

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
14539 14535 14527 14525 14534 14533 14535 14535 14535 14539 14537 14536 14528 14544 14545
14526 14526 14535 14534 14526 14528 14534 14526 14534 14532 14534 14534 14534 14530 14541
14535 14535 14540 14533 14533 14533 14533 14527 14532 14533 14534 14532 14526 14536 14539
14530 14535 14527 14532 14533 14527 14533 14528 14532 14534 14532 14533 14525 14525 14538
14528 14528 14528 14532 14535 14535 14526 14526 14532 14532 14534 14531 14525 14527 14540
14533 14527 14535 14528 14521 14533 14528 14527 14536 14535 14534 14534 14528 14532 14536
14528 14534 14531 14534 14533 14528 14527 14534 14532 14533 14528 14535 14527 14526 14532
14528 14538 14535 14532 14534 14533 14533 14526 14537 14533 14534 14534 14531 14535 14540
14535 14535 14530 14532 14535 14528 14533 14526 14533 14535 14532 14532 14532 14534 14537
14540 14533 14530 14533 14533 14536 14533 14527 14533 14532 14534 14534 14525 14527 14535
14528 14527 14527 14533 14535 14535 14527 14527 14533 14533 14532 14534 14525 14543 14536
14533 14536 14534 14534 14522 14533 14528 14527 14533 14533 14533 14533 14533 14543 14537
14534 14533 14527 14534 14533 14536 14534 14527 14532 14533 14533 14533 14535 14534 14536
14526 14529 14539 14532 14533 14528 14533 14528 14533 14532 14529 14532 14525 14528 14538
14527 14528 14531 14534 14533 14532 14532 14527 14531 14532 14532 14534 14525 14541 14538
14540 14538 14539 14534 14534 14527 14533 14525 14532 14533 14528 14532 14531 14542 14534
14529 14536 14525 14532 14533 14527 14528 14527 14536 14534 14533 14531 14526 14526 14538
14528 14528 14539 14533 14533 14528 14533 14526 14532 14533 14533 14534 14529 14526 14536
14527 14535 14535 14532 14533 14528 14533 14529 14532 14536 14533 14532 14533 14535 14538
14528 14526 14532 14536 14534 14528 14533 14528 14533 14534 14526 14534 14526 14534 14538
14528 14529 14530 14534 14533 14527 14533 14527 14532 14533 14532 14534 14529 14526 14537
14528 14527 14535 14534 14533 14527 14534 14527 14532 14532 14533 14532 14529 14542 14537
14533 14527 14525 14534 14534 14526 14528 14527 14531 14533 14533 14533 14526 14536 14536

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 14527 bytes 100%
1,000 14524 bytes -3 bytes 100%
10,000 14522 bytes -2 bytes 100%
100,000 14521 bytes -1 byte 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
14591 bytes +70 bytes (+0.48%) +49 bytes
14591 bytes +70 bytes (+0.48%) +49 bytes
14542 bytes +21 bytes (+0.14%)
14556 bytes +35 bytes (+0.24%) +14 bytes
14594 bytes +73 bytes (+0.50%) +52 bytes
14628 bytes +107 bytes (+0.74%) +86 bytes
14660 bytes +139 bytes (+0.96%) +118 bytes
14698 bytes +177 bytes (+1.22%) +156 bytes
14733 bytes +212 bytes (+1.46%) +191 bytes

Non-DEFLATE Algorithms

Archivers based on completely different compression algorithms often produce superior results.
Unfortunately, browsers only support gzip compression at the moment.
Algorithm Program Parameters Size Compared To Best Zopfli
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 12526 bytes -1995 bytes (-13.74%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 13042 bytes -1479 bytes (-10.19%)
Brotli (Wikipedia) brotli brotli -q 11 13320 bytes -1201 bytes (-8.27%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 13395 bytes -1126 bytes (-7.75%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14026 bytes -495 bytes (-3.41%)
LZMA2 (Wikipedia) xz xz -9 14388 bytes -133 bytes (-0.92%)
ZSTD (Wikipedia) zstd zstd -19 14540 bytes +19 bytes (+0.13%)

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):


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 2018.
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