Choose a version:
21% The original file has 691206 bytes (675.0k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 147398 bytes (143.9k, 21%).

After GZIP compression these minified files vary in size:
Boot
  51374 bytes (50.2k)
CDN
cdnjs
  44197 bytes (43.2k)
CDN
cdnhttps
  43783 bytes (42.8k)
CDN
unpkg
  43783 bytes (42.8k)
CDN
gzip -6 (default)
  43771 bytes (42.7k)
local copy
jsdelivr
  43699 bytes (42.7k)
CDN
gzip -9
  43615 bytes (42.6k)
local copy
7zip -mx=9 -tgzip
  42170 bytes (41.2k)
local copy
libdeflate -12
  42145 bytes (41.2k)
local copy
kzip -s0 -rn -b7
  42064 bytes (41.1k)
local copy
pigz -11 -n
  42029 bytes (41.0k)
local copy
Zopfli
  41885 bytes (40.9k)
local copy
Zopfli (defluff)
  41884 bytes (40.9k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/react/react-15.2.1.min.js (or via HTTPS)

You will automatically get the smallest React 15.2.1 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 1814 bytes by using my React 15.2.1 Zopfli version instead of the best available CDN (4.33% smaller than jsdelivr, 41885 vs. 43699 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 --i1000000 --mb8 --mls32 --bsr30 --lazy --ohh

(found July 12, 2016)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 32  --mls32
block splitting recursion 30  --bsr30
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 (41884 bytes).

Verify file integrity

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

MD5:
curl --silent --compressed https://fb.me/react-15.2.1.min.js --location | md5sum
a4137323c75e65beca5a3ca602d78a87  -
curl --silent --compressed https://minime.stephan-brumme.com/files/react/react-15.2.1.min.zopfli.js.gz | md5sum
a4137323c75e65beca5a3ca602d78a87  -

SHA1:
curl --silent --compressed https://fb.me/react-15.2.1.min.js --location | sha1sum
fb5f28b05620fea85160d7ca4ea2cc5a7120d41b  -
curl --silent --compressed https://minime.stephan-brumme.com/files/react/react-15.2.1.min.zopfli.js.gz | sha1sum
fb5f28b05620fea85160d7ca4ea2cc5a7120d41b  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 51374 bytes a4137323c75e65beca5a3ca602d78a87 July 10, 2016 @ 09:02
cdnjs 44197 bytes a4137323c75e65beca5a3ca602d78a87 July 9, 2016 @ 10:16
cdnhttps 43783 bytes a4137323c75e65beca5a3ca602d78a87 November 22, 2016 @ 16:40
unpkg 43783 bytes a4137323c75e65beca5a3ca602d78a87 July 11, 2016 @ 15:32
jsdelivr 43699 bytes a4137323c75e65beca5a3ca602d78a87 July 9, 2016 @ 00:35

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

Other Versions

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

16.5.2, 16.5.1, 16.5.0,
16.4.2, 16.4.1, 16.4.0,
16.3.2, 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
41885 bytes -1 byte zopfli --i1000000 --mls32 --bsr30 --lazy --ohh July 12, 2016 @ 12:31
41886 bytes -2 bytes zopfli --i1000000 --mls64 --bsr17 --lazy --ohh July 12, 2016 @ 01:59
41888 bytes -4 bytes zopfli --i100000 --mls64 --bsr17 --lazy --ohh July 11, 2016 @ 12:08
41892 bytes -1 byte zopfli --i10000 --mls64 --bsr17 --lazy --ohh July 11, 2016 @ 11:14
41893 bytes -8 bytes zopfli --i10000 --mls32 --bsr30 --lazy --ohh July 11, 2016 @ 11:09
41901 bytes -1 byte zopfli --i1000 --mls64 --bsr17 --lazy --ohh July 11, 2016 @ 11:05
41902 bytes -11 bytes zopfli --i1000 --mls32 --bsr30 --lazy --ohh July 11, 2016 @ 11:04
41913 bytes zopfli --i100 --mls32 --bsr30 --lazy --ohh July 11, 2016 @ 11:00

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, 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
41946 42099 41980 41966 41957 41962 42099 42099 42098 42192 42190 42195 42110 42110 42102
41928 41937 41995 41937 41907 41913 41943 41920 41989 41914 41997 42098 42098 42111 42110
41991 41909 42106 42000 41998 42094 42000 41996 42096 42101 42005 42100 42109 42110 42112
41911 41909 41925 41910 41916 41987 41910 41992 42096 42096 41987 42097 42103 42099 42110
41926 41923 41924 41935 41929 41926 41908 42100 41908 41914 41907 42099 42107 42104 42107
41914 42101 41995 41997 42006 42095 41991 42013 42097 41910 41991 42098 42105 42104 42107
41998 42102 42106 41992 41998 41906 42000 41986 41990 41991 41992 42096 42105 42107 42105
41913 41926 41923 41915 42005 41908 41910 42013 41910 41910 41909 42096 42104 42107 42112
41912 41912 42099 41991 41997 41998 42005 41996 42100 41909 41985 42097 42113 42105 42106
41920 41914 41915 41914 41916 41913 41907 41992 41910 41910 41917 42096 42107 42105 42107
41922 41925 41930 42007 41907 41912 41909 41990 41991 41909 41906 42098 42105 42105 42104
41914 41990 41913 41995 41988 41993 41995 41993 41990 41987 41901 42097 42107 42105 42112
41914 41912 41913 41903 41913 41909 41910 41911 41910 41903 41912 42097 42106 42108 42108
41934 41932 41910 41935 41992 41886 41994 41997 41910 41904 41990 42097 42105 42105 42109
41918 41913 41912 41994 41908 41904 41907 41911 41908 41914 41912 42097 42104 42105 42107
41912 41994 41990 41997 41909 41911 41908 41911 41910 41907 42095 42098 42104 42112 42113
41994 41911 41909 41911 41892 41906 41989 41988 41989 41910 41907 42103 42104 42108 42109
41917 41913 41910 41902 41907 41891 41908 41909 41907 41909 41912 42097 42104 42103 42115
41914 41912 41911 41993 41996 41912 41911 41910 41917 41910 41992 42096 42104 42105 42115
41924 41906 41909 41917 41995 41895 41907 41908 41908 41986 41906 42098 42104 42103 42111
41912 41913 41910 41908 41893 41910 41911 41986 41991 41908 41914 42097 42106 42105 42108
41911 41910 41912 41995 41885 41910 41910 41906 41905 41911 41914 42099 42105 42105 42109
41917 41914 41990 41893 41910 41903 41911 41908 41910 41907 41913 42097 42107 42105 42106

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 41913 bytes 100%
1,000 41901 bytes -12 bytes 100%
10,000 41892 bytes -9 bytes 100%
100,000 41888 bytes -4 bytes 2.03%
1,000,000 41885 bytes -3 bytes 0.58%
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
42204 bytes +319 bytes (+0.76%) +140 bytes
42198 bytes +313 bytes (+0.75%) +134 bytes
42205 bytes +320 bytes (+0.76%) +141 bytes
42126 bytes +241 bytes (+0.58%) +62 bytes
42087 bytes +202 bytes (+0.48%) +23 bytes
42097 bytes +212 bytes (+0.51%) +33 bytes
42075 bytes +190 bytes (+0.45%) +11 bytes
42064 bytes +179 bytes (+0.43%)
42100 bytes +215 bytes (+0.51%) +36 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 - but your browser doesn't support it.
Algorithm Program Parameters Size Compared To Best Zopfli
ZPAQ (Wikipedia) zpaq zpaq -method 69 30659 bytes -11226 bytes (-26.80%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 33206 bytes -8679 bytes (-20.72%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 35295 bytes -6590 bytes (-15.73%)
Brotli (Wikipedia) brotli brotli -q 11 36955 bytes -4930 bytes (-11.77%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 37514 bytes -4371 bytes (-10.44%)
LZMA2 (Wikipedia) xz xz -9 38160 bytes -3725 bytes (-8.89%)
Zstandard (Wikipedia) zstd zstd -19 39822 bytes -2063 bytes (-4.93%)

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