Choose a version:
17% The original file has 127517 bytes (124.5k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 21339 bytes (20.8k, 17%).

After GZIP compression these minified files vary in size:
Boot
  8213 bytes (8.0k)
CDN
cdnjs
  7359 bytes (7.2k)
CDN
gzip -6 (default)
  7357 bytes (7.2k)
local copy
gzip -9
  7336 bytes (7.2k)
local copy
unpkg
  7334 bytes (7.2k)
CDN
jsdelivr
  7333 bytes (7.2k)
CDN
libdeflate -12
  7150 bytes (7.0k)
local copy
7zip -mx=9 -tgzip
  7118 bytes (7.0k)
local copy
pigz -11 -n
  7062 bytes (6.9k)
local copy
kzip -s0 -rn -b3
  7049 bytes (6.9k)
local copy
Zopfli
  7028 bytes (6.9k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/react/react-15.4.1.min.js

You will automatically get the smallest React 15.4.1 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 305 bytes by using my React 15.4.1 Zopfli version instead of the best available CDN (4.34% smaller than jsdelivr, 7028 vs. 7333 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 --bsr9 --lazy --ohh

(found November 23, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 128  --mls128
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

Verify file integrity

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

MD5:
curl --silent --compressed https://fb.me/react-15.4.1.min.js --location | md5sum
c729daecb8c864a0108d0af778bb24a1  -
curl --silent --compressed http://minime.stephan-brumme.com/files/react/react-15.4.1.min.zopfli.js.gz | md5sum
c729daecb8c864a0108d0af778bb24a1  -

SHA1:
curl --silent --compressed https://fb.me/react-15.4.1.min.js --location | sha1sum
8b2dca1db47189b74cec02ac400b8e842951deac  -
curl --silent --compressed http://minime.stephan-brumme.com/files/react/react-15.4.1.min.zopfli.js.gz | sha1sum
8b2dca1db47189b74cec02ac400b8e842951deac  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 8213 bytes c729daecb8c864a0108d0af778bb24a1 December 3, 2016 @ 14:26
cdnjs 7359 bytes c729daecb8c864a0108d0af778bb24a1 December 3, 2016 @ 04:32
unpkg 7334 bytes c729daecb8c864a0108d0af778bb24a1 November 23, 2016 @ 10:12
jsdelivr 7333 bytes c729daecb8c864a0108d0af778bb24a1 (invalid)

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
7028 bytes -1 byte zopfli --i100000 --mls128 --bsr9 --lazy --ohh November 23, 2016 @ 15:24
7029 bytes -1 byte zopfli --i10000 --mls128 --bsr9 --lazy --ohh November 23, 2016 @ 11:45
7030 bytes -1 byte zopfli --i10000 --mls512 --bsr9 --lazy --ohh November 23, 2016 @ 11:37
7031 bytes -3 bytes zopfli --i1000 --mls256 --bsr11 --lazy --ohh November 23, 2016 @ 11:13
7034 bytes zopfli --i100 --mls256 --bsr11 --lazy --ohh November 23, 2016 @ 11:10

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 @ 15:27.

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
7102 7102 7107 7061 7074 7070 7061 7135 7058 7073 7139 7063 7071 7079 7079
7062 7061 7064 7067 7065 7062 7066 7068 7076 7055 7082 7066 7052 7044 7043
7041 7032 7046 7044 7041 7047 7043 7042 7042 7040 7053 7055 7089 7089 7053
7037 7037 7059 7032 7052 7049 7033 7032 7049 7056 7049 7047 7132 7075 7075
7040 7040 7034 7034 7047 7034 7058 7043 7052 7059 7053 7054 7131 7077 7079
7042 7042 7035 7033 7038 7028 7028 7034 7029 7052 7055 7038 7137 7083 7036
7035 7067 7036 7037 7034 7031 7075 7075 7066 7053 7054 7057 7055 7082 7090
7039 7043 7036 7035 7035 7033 7028 7029 7033 7028 7032 7031 7131 7044 7079
7039 7036 7035 7054 7054 7047 7029 7035 7049 7039 7052 7042 7131 7051 7082
7037 7042 7056 7033 7028 7028 7032 7033 7028 7028 7053 7053 7132 7061 7058
7041 7037 7058 7032 7039 7032 7028 7033 7032 7054 7052 7033 7053 7090 7043
7037 7036 7059 7033 7032 7031 7033 7033 7054 7058 7052 7054 7131 7040 7041
7036 7037 7039 7038 7040 7033 7032 7040 7041 7053 7053 7132 7131 7061 7056
7040 7038 7036 7033 7038 7032 7028 7034 7082 7053 7136 7056 7136 7084 7053
7037 7037 7036 7032 7041 7028 7028 7033 7032 7031 7049 7056 7137 7083 7083
7038 7037 7038 7037 7041 7033 7032 7033 7028 7053 7052 7043 7132 7039 7039
7039 7037 7038 7032 7033 7031 7028 7032 7033 7028 7054 7045 7050 7089 7053
7038 7041 7036 7038 7041 7031 7028 7035 7028 7054 7048 7053 7053 7087 7059
7042 7039 7038 7037 7032 7032 7032 7034 7072 7054 7053 7053 7137 7039 7041
7038 7039 7037 7033 7032 7034 7037 7036 7032 7032 7033 7037 7132 7040 7040
7037 7033 7034 7038 7041 7033 7033 7032 7028 7028 7055 7032 7137 7080 7053
7036 7040 7035 7034 7040 7034 7033 7036 7028 7028 7031 7033 7052 7053 7053
7040 7039 7034 7032 7041 7031 7033 7032 7032 7032 7053 7131 7131 7091 7054

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 7034 bytes 100%
1,000 7031 bytes -3 bytes 100%
10,000 7029 bytes -2 bytes 100%
100,000 7028 bytes -1 byte 6.96%
1,000,000 7028 bytes 6.09%
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
7144 bytes +116 bytes (+1.65%) +95 bytes
7143 bytes +115 bytes (+1.64%) +94 bytes
7135 bytes +107 bytes (+1.52%) +86 bytes
7049 bytes +21 bytes (+0.30%)
7072 bytes +44 bytes (+0.63%) +23 bytes
7103 bytes +75 bytes (+1.07%) +54 bytes
7137 bytes +109 bytes (+1.55%) +88 bytes
7171 bytes +143 bytes (+2.03%) +122 bytes
7197 bytes +169 bytes (+2.40%) +148 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 6427 bytes -601 bytes (-8.55%)
Brotli (Wikipedia) brotli brotli -q 11 6487 bytes -541 bytes (-7.70%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 6872 bytes -156 bytes (-2.22%)
LZMA2 (Wikipedia) xz xz -9 6940 bytes -88 bytes (-1.25%)
ZSTD (Wikipedia) zstd zstd -19 7076 bytes +48 bytes (+0.68%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 7103 bytes +75 bytes (+1.07%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 7215 bytes +187 bytes (+2.66%)

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