Choose a version:
22% The original file has 655607 bytes (640.2k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 145410 bytes (142.0k, 22%).

After GZIP compression these minified files vary in size:
Boot
  50542 bytes (49.4k)
CDN
cdnjs
  43302 bytes (42.3k)
CDN
cdnhttps
  42865 bytes (41.9k)
CDN
unpkg
  42865 bytes (41.9k)
CDN
gzip -6 (default)
  42859 bytes (41.9k)
local copy
jsdelivr
  42781 bytes (41.8k)
CDN
gzip -9
  42681 bytes (41.7k)
local copy
7zip -mx=9 -tgzip
  41254 bytes (40.3k)
local copy
libdeflate -12
  41220 bytes (40.3k)
local copy
kzip -s0 -rn -b7
  41156 bytes (40.2k)
local copy
pigz -11 -n
  41121 bytes (40.2k)
local copy
Zopfli
  40973 bytes (40.0k)
local copy
Zopfli (defluff)
  40971 bytes (40.0k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/react/react-15.0.0.min.js

You will automatically get the smallest React 15.0.0 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 1808 bytes by using my React 15.0.0 Zopfli version instead of the best available CDN (4.41% smaller than jsdelivr, 40973 vs. 42781 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 --mls128 --bsr25 --lazy --ohh

(found April 9, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 128  --mls128
block splitting recursion 25  --bsr25
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 2 more bytes (40971 bytes).

Verify file integrity

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

MD5:
curl --silent --compressed https://fb.me/react-15.0.0.min.js --location | md5sum
1456b8de38f7c496c82b3ed8cdc0615f  -
curl --silent --compressed http://minime.stephan-brumme.com/files/react/react-15.0.0.min.zopfli.js.gz | md5sum
1456b8de38f7c496c82b3ed8cdc0615f  -

SHA1:
curl --silent --compressed https://fb.me/react-15.0.0.min.js --location | sha1sum
00b5c5d695dc48ca15a9259dcb3f1c95a7cb7be4  -
curl --silent --compressed http://minime.stephan-brumme.com/files/react/react-15.0.0.min.zopfli.js.gz | sha1sum
00b5c5d695dc48ca15a9259dcb3f1c95a7cb7be4  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 50542 bytes 1456b8de38f7c496c82b3ed8cdc0615f May 6, 2016 @ 13:19
cdnjs 43302 bytes 1456b8de38f7c496c82b3ed8cdc0615f April 7, 2016 @ 23:46
cdnhttps 42865 bytes 1456b8de38f7c496c82b3ed8cdc0615f November 22, 2016 @ 16:40
unpkg 42865 bytes 1456b8de38f7c496c82b3ed8cdc0615f July 11, 2016 @ 15:33
jsdelivr 42781 bytes 1456b8de38f7c496c82b3ed8cdc0615f April 7, 2016 @ 23:36

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

Other Versions

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

16.3.1, 16.3.0,
16.2.0,
16.1.1, 16.1.0, 16.0.0,
15.6.2, 15.6.1, 15.6.0,
15.5.2, 15.5.1, 15.5.0,
15.4.2, 15.4.1, 15.4.0,
15.3.2, 15.3.1, 15.3.0,
15.2.1, 15.2.0,
15.1.0,
15.0.2, 15.0.1, 15.0.0,
0.14.8, 0.14.7, 0.14.6, 0.14.5, 0.14.4, 0.14.3, 0.14.2, 0.14.1, 0.14.0,
0.13.3, 0.13.2, 0.13.1, 0.13.0,
0.12.2, 0.12.1, 0.12.0,
0.11.2, 0.11.1, 0.11.0,
0.10.0

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, Socket.IO, ThreeJS, UnderscoreJS and Vue.

Changelog

Best Zopfli parameters so far:
Size Improvement Parameters Found
40973 bytes -4 bytes zopfli --i100000 --mls128 --bsr25 --lazy --ohh April 9, 2016 @ 00:57
40977 bytes -3 bytes zopfli --i100000 --mls64 --bsr25 --lazy --ohh April 8, 2016 @ 18:08
40980 bytes -8 bytes zopfli --i10000 --mls64 --bsr25 --lazy --ohh April 8, 2016 @ 17:06
40988 bytes -3 bytes zopfli --i1000 --mls64 --bsr25 --lazy --ohh April 8, 2016 @ 17:00
40991 bytes -14 bytes zopfli --i1000 --mls32 --bsr30 --lazy --ohh April 8, 2016 @ 16:53
41005 bytes zopfli --i100 --mls32 --bsr30 --lazy --ohh April 8, 2016 @ 16:50

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

Most recent activity on November 22, 2016 @ 16:40.

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
41065 41189 41190 41191 41190 41189 41191 41188 41190 41189 41187 41185 41185 41192 41203
41104 41089 41103 41105 40991 41093 41094 41087 41097 41099 40998 41193 41193 41028 41097
41190 41189 41186 41101 41183 41185 41188 41091 41187 41191 41188 41186 41196 41021 41005
41003 41185 41082 41090 41086 41084 41182 41088 41185 40998 41002 41186 41194 41007 41012
41012 41003 41001 41000 40990 41006 40976 41009 40998 40995 41001 41187 41196 41015 41013
41003 41086 41089 41004 41017 41013 41083 41084 41086 41081 40997 41186 41197 41009 41093
41000 41189 41088 41084 41080 41087 41182 41082 41186 41184 41182 41186 41198 41017 41196
41018 40998 41001 41005 41004 41002 40985 41002 41005 40997 40999 41185 41195 41013 41098
41006 41007 41002 41007 41008 40988 40985 41005 40997 40997 41000 41186 41012 41010 41009
41092 41190 41089 41087 41085 41188 41185 41084 41182 41081 41182 41185 41198 41194 41011
41008 41088 41089 41007 41001 41085 41081 41087 41086 40998 41185 41186 41196 41008 41095
41003 40997 41000 40997 41010 40977 40985 40996 41000 40995 41001 41185 41196 41008 41095
41006 41002 41000 40998 41000 41006 41003 41002 41001 40996 41002 41184 41195 41012 41014
41003 41002 41001 41005 41001 41005 41008 40995 41000 40993 40999 41184 41196 41009 41010
41005 41088 41083 41004 41006 41089 41094 41093 41083 41082 41001 41184 41196 41010 41016
41007 41003 41189 41088 41088 41086 41182 41188 40999 40997 40995 41186 41197 41013 41014
41012 41002 41089 41089 41085 41081 41182 41079 41181 40996 41183 41186 41196 41009 41011
41008 41007 41004 41005 41003 40996 41001 41080 41003 40996 41000 41186 41195 41009 41009
41009 41006 41004 41005 40991 40986 40998 41005 40998 41003 40995 41186 41196 41010 41098
41008 41087 41004 41003 41000 41003 41006 41086 41003 40999 41002 41185 41195 41008 41098
41005 41006 41002 41090 41006 40977 40973 41008 41088 40995 41002 41185 41194 41009 41010
41014 40998 41006 41004 40986 41005 41084 41088 41002 40996 41000 41184 41196 41011 41010
41004 41003 41002 41001 41011 41001 40978 41005 41003 40996 41006 41186 41196 41020 41094

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 41005 bytes 100%
1,000 40988 bytes -17 bytes 100%
10,000 40979 bytes -9 bytes 100%
100,000 40973 bytes -6 bytes 1.45%
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
41281 bytes +308 bytes (+0.75%) +125 bytes
41278 bytes +305 bytes (+0.74%) +122 bytes
41283 bytes +310 bytes (+0.76%) +127 bytes
41218 bytes +245 bytes (+0.60%) +62 bytes
41179 bytes +206 bytes (+0.50%) +23 bytes
41188 bytes +215 bytes (+0.52%) +32 bytes
41168 bytes +195 bytes (+0.48%) +12 bytes
41156 bytes +183 bytes (+0.45%)
41169 bytes +196 bytes (+0.48%) +13 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
ZPAQ (Wikipedia) zpaq zpaq -method 69 30005 bytes -10968 bytes (-26.77%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 32496 bytes -8477 bytes (-20.69%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 34623 bytes -6350 bytes (-15.50%)
Brotli (Wikipedia) brotli brotli -q 11 36111 bytes -4862 bytes (-11.87%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 36740 bytes -4233 bytes (-10.33%)
LZMA2 (Wikipedia) xz xz -9 37348 bytes -3625 bytes (-8.85%)
ZSTD (Wikipedia) zstd zstd -19 38960 bytes -2013 bytes (-4.91%)

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