Choose a version:
17% The original file has 127694 bytes (124.7k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 21203 bytes (20.7k, 17%).

After GZIP compression these minified files vary in size:
Boot
  8163 bytes (8.0k)
CDN, direct link
cdnjs
  7326 bytes (7.2k)
CDN, direct link
gzip -6 (default)
  7325 bytes (7.2k)
local copy
gzip -9
  7306 bytes (7.1k)
local copy
unpkg
  7305 bytes (7.1k)
CDN, direct link
jsdelivr
  7303 bytes (7.1k)
CDN, direct link
libdeflate -12
  7121 bytes (7.0k)
local copy
7zip -mx=9 -tgzip
  7076 bytes (6.9k)
local copy
pigz -11 -n
  7035 bytes (6.9k)
local copy
kzip -s0 -rn -b3
  7024 bytes (6.9k)
local copy
Zopfli
  7002 bytes (6.8k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/react/react-15.4.2.min.js

You will automatically get the smallest React 15.4.2 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 301 bytes by using my React 15.4.2 Zopfli version instead of the best available CDN (4.30% smaller than jsdelivr, 7002 vs. 7303 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 --mls4 --bsr25 --lazy --ohh

(found January 9, 2017)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 4  --mls4
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

Verify file integrity

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

MD5:
curl --silent --compressed https://fb.me/react-15.4.2.min.js --location | md5sum
fe3e04b0a969e3722ae34c9a06ef8f32  -
curl --silent --compressed http://minime.stephan-brumme.com/files/react/react-15.4.2.min.zopfli.js.gz | md5sum
fe3e04b0a969e3722ae34c9a06ef8f32  -

SHA1:
curl --silent --compressed https://fb.me/react-15.4.2.min.js --location | sha1sum
b71145e063c22556d375e065e02ea2ac66fd606a  -
curl --silent --compressed http://minime.stephan-brumme.com/files/react/react-15.4.2.min.zopfli.js.gz | sha1sum
b71145e063c22556d375e065e02ea2ac66fd606a  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 8163 bytes fe3e04b0a969e3722ae34c9a06ef8f32 January 10, 2017 @ 05:20
cdnjs 7326 bytes fe3e04b0a969e3722ae34c9a06ef8f32 (invalid)
unpkg 7305 bytes fe3e04b0a969e3722ae34c9a06ef8f32 January 9, 2017 @ 14:46
jsdelivr 7303 bytes fe3e04b0a969e3722ae34c9a06ef8f32 (invalid)

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

Other Versions

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

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
7002 bytes -2 bytes zopfli --i100000 --mls4 --bsr25 --lazy --ohh January 9, 2017 @ 15:31
7004 bytes -1 byte zopfli --i1000 --mls32 --bsr16 --lazy --ohh January 9, 2017 @ 14:55
7005 bytes -1 byte zopfli --i1000 --mls16 --bsr9 --lazy --ohh January 9, 2017 @ 14:55
7006 bytes zopfli --i100 --mls32 --bsr10 --lazy --ohh January 9, 2017 @ 14:47

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

Most recent activity on January 25, 2017 @ 15:29.

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, 100,000 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
7035 7035 7054 7051 7029 7030 7030 7038 7040 7039 7108 7106 7109 7085 7087
7029 7030 7043 7035 7038 7042 7044 7046 7026 7037 7029 7015 7104 7046 7022
7013 7012 7021 7017 7003 7013 7014 7013 7012 7016 7012 7013 7106 7017 7023
7007 7015 7008 7006 7023 7026 7003 7007 7046 7024 7110 7013 7018 7056 7027
7049 7034 7029 7025 7009 7028 7028 7029 7025 7024 7025 7015 7104 7015 7022
7013 7014 7010 7003 7006 7007 7006 7003 7003 7026 7108 7018 7018 7017 7017
7039 7034 7010 7026 7003 7015 7028 7010 7101 7015 7025 7023 7025 7056 7024
7011 7010 7012 7007 7003 7015 7003 7006 7029 7049 7007 7013 7104 7016 7026
7012 7102 7007 7025 7026 7014 7009 7011 7103 7024 7110 7016 7018 7047 7029
7014 7010 7005 7009 7003 7008 7003 7007 7006 7007 7006 7030 7033 7057 7025
7012 7014 7010 7005 7006 7007 7006 7003 7006 7027 7110 7029 7103 7053 7013
7052 7027 7007 7007 7006 7006 7003 7007 7024 7006 7007 7018 7103 7052 7026
7015 7009 7005 7003 7003 7012 7006 7012 7025 7026 7103 7015 7104 7025 7013
7036 7027 7005 7006 7003 7005 7003 7007 7025 7027 7022 7016 7029 7053 7053
7015 7007 7005 7006 7006 7006 7006 7003 7006 7004 7009 7013 7029 7059 7026
7015 7006 7008 7003 7003 7006 7003 7006 7003 7026 7022 7019 7104 7045 7026
7007 7010 7008 7006 7003 7005 7006 7014 7003 7006 7022 7016 7103 7052 7022
7015 7008 7008 7006 7003 7005 7006 7003 7024 7027 7110 7028 7027 7052 7024
7012 7009 7008 7008 7009 7011 7006 7003 7003 7006 7008 7023 7034 7017 7026
7007 7015 7009 7007 7007 7008 7006 7007 7009 7006 7009 7009 7018 7017 7026
7006 7002 7007 7007 7003 7004 7007 7005 7003 7009 7104 7013 7017 7033 7059
7007 7017 7006 7008 7007 7007 7003 7003 7006 7007 7006 7010 7104 7045 7034
7012 7010 7008 7006 7005 7006 7006 7030 7048 7026 7108 7013 7103 7012 7027

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 7006 bytes 100%
1,000 7004 bytes -2 bytes 100%
10,000 7004 bytes 100%
100,000 7002 bytes -2 bytes 9.86%
1,000,000 7002 bytes 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
7115 bytes +113 bytes (+1.61%) +91 bytes
7115 bytes +113 bytes (+1.61%) +91 bytes
7109 bytes +107 bytes (+1.53%) +85 bytes
7024 bytes +22 bytes (+0.31%)
7043 bytes +41 bytes (+0.59%) +19 bytes
7079 bytes +77 bytes (+1.10%) +55 bytes
7109 bytes +107 bytes (+1.53%) +85 bytes
7142 bytes +140 bytes (+2.00%) +118 bytes
7167 bytes +165 bytes (+2.36%) +143 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 6400 bytes -602 bytes (-8.60%)
Brotli (Wikipedia) brotli brotli -q 11 6493 bytes -509 bytes (-7.27%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 6851 bytes -151 bytes (-2.16%)
LZMA2 (Wikipedia) xz xz -9 6916 bytes -86 bytes (-1.23%)
ZSTD (Wikipedia) zstd zstd -19 7045 bytes +43 bytes (+0.61%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 7080 bytes +78 bytes (+1.11%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 7196 bytes +194 bytes (+2.77%)

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 2017.
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