Choose a version:
22% The original file has 656686 bytes (641.3k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 145685 bytes (142.3k, 22%).

After GZIP compression these minified files vary in size:
Boot
  50662 bytes (49.5k)
CDN, direct link
jsdelivr
  50662 bytes (49.5k)
CDN, direct link
cdnjs
  43381 bytes (42.4k)
CDN, direct link
cdnhttps
  42945 bytes (41.9k)
CDN, direct link
unpkg
  42945 bytes (41.9k)
CDN, direct link
gzip -6 (default)
  42940 bytes (41.9k)
local copy
gzip -9
  42763 bytes (41.8k)
local copy
7zip -mx=9 -tgzip
  41330 bytes (40.4k)
local copy
libdeflate -12
  41299 bytes (40.3k)
local copy
kzip -s0 -rn -b7
  41232 bytes (40.3k)
local copy
pigz -11 -n
  41202 bytes (40.2k)
local copy
Zopfli
  41047 bytes (40.1k)
local copy
Zopfli (defluff)
  41045 bytes (40.1k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/react/react-15.0.1.min.js

You will automatically get the smallest React 15.0.1 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 1898 bytes by using my React 15.0.1 Zopfli version instead of the best available CDN (4.62% smaller than unpkg, 41047 vs. 42945 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 --mls64 --bsr12 --lazy --ohh

(found April 9, 2016)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 64  --mls64
block splitting recursion 12  --bsr12
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 (41045 bytes).

Verify file integrity

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

MD5:
curl --silent --compressed https://fb.me/react-15.0.1.min.js --location | md5sum
075a24757ddf994ee088ae96f4d5682e  -
curl --silent --compressed http://minime.stephan-brumme.com/files/react/react-15.0.1.min.zopfli.js.gz | md5sum
075a24757ddf994ee088ae96f4d5682e  -

SHA1:
curl --silent --compressed https://fb.me/react-15.0.1.min.js --location | sha1sum
fc6d66d9b489eee1bbe04d744bf1ad84346f9a95  -
curl --silent --compressed http://minime.stephan-brumme.com/files/react/react-15.0.1.min.zopfli.js.gz | sha1sum
fc6d66d9b489eee1bbe04d744bf1ad84346f9a95  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 50662 bytes 075a24757ddf994ee088ae96f4d5682e May 6, 2016 @ 13:19
jsdelivr 50662 bytes 075a24757ddf994ee088ae96f4d5682e April 8, 2016 @ 20:36
cdnjs 43381 bytes 075a24757ddf994ee088ae96f4d5682e April 8, 2016 @ 20:46
cdnhttps 42945 bytes 075a24757ddf994ee088ae96f4d5682e November 22, 2016 @ 16:40
unpkg 42945 bytes 075a24757ddf994ee088ae96f4d5682e July 11, 2016 @ 15:32

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
41047 bytes -3 bytes zopfli --i1000000 --mls64 --bsr12 --lazy --ohh April 9, 2016 @ 15:11
41050 bytes -5 bytes zopfli --i100000 --mls64 --bsr12 --lazy --ohh April 9, 2016 @ 01:32
41055 bytes -9 bytes zopfli --i10000 --mls64 --bsr12 --lazy --ohh April 9, 2016 @ 00:27
41064 bytes -13 bytes zopfli --i1000 --mls64 --bsr12 --lazy --ohh April 9, 2016 @ 00:20
41077 bytes zopfli --i100 --mls64 --bsr12 --lazy --ohh April 9, 2016 @ 00:17

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
41156 41265 41264 41263 41264 41265 41262 41267 41262 41265 41264 41263 41262 41263 41276
41181 41180 41179 41095 41104 41086 41106 41107 41110 41170 41101 41098 41269 41090 41170
41082 41263 41172 41174 41268 41264 41259 41169 41160 41263 41259 41259 41271 41098 41088
41084 41073 41163 41164 41157 41161 41161 41158 41159 41075 41259 41077 41269 41104 41087
41089 41079 41076 41081 41067 41076 41075 41084 41074 41075 41071 41260 41088 41093 41086
41078 41160 41162 41160 41078 41163 41160 41158 41171 41158 41070 41260 41089 41085 41170
41162 41167 41164 41167 41158 41166 41160 41160 41168 41076 41258 41260 41269 41269 41272
41082 41074 41077 41078 41076 41166 41077 41080 41074 41084 41073 41261 41088 41085 41083
41076 41090 41169 41159 41078 41047 41075 41075 41076 41074 41082 41073 41083 41080 41084
41080 41162 41173 41162 41162 41165 41161 41155 41165 41077 41260 41260 41271 41269 41094
41084 41162 41159 41160 41080 41164 41154 41160 41161 41159 41077 41260 41091 41085 41170
41080 41076 41077 41080 41066 41077 41074 41080 41074 41075 41080 41259 41273 41087 41175
41082 41079 41076 41161 41066 41081 41072 41082 41075 41075 41080 41260 41091 41087 41176
41084 41076 41078 41057 41162 41072 41074 41080 41074 41079 41080 41260 41083 41086 41087
41080 41159 41156 41081 41073 41165 41163 41159 41163 41164 41070 41259 41087 41083 41100
41079 41163 41162 41164 41163 41077 41163 41081 41076 41078 41074 41259 41085 41091 41172
41079 41078 41161 41165 41162 41078 41161 41161 41072 41076 41259 41261 41270 41084 41171
41077 41089 41076 41078 41078 41074 41078 41157 41076 41073 41078 41260 41088 41084 41086
41085 41084 41073 41082 41060 41056 41075 41075 41076 41077 41072 41260 41083 41084 41083
41084 41081 41080 41081 41159 41059 41076 41082 41078 41075 41073 41260 41085 41083 41176
41082 41076 41163 41085 41079 41071 41081 41079 41083 41075 41080 41260 41090 41089 41090
41081 41071 41075 41083 41085 41076 41158 41161 41160 41161 41073 41260 41088 41087 41085
41085 41077 41078 41080 41078 41081 41073 41079 41078 41078 41073 41259 41091 41085 41086

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 41077 bytes 100%
1,000 41064 bytes -13 bytes 100%
10,000 41055 bytes -9 bytes 100%
100,000 41050 bytes -5 bytes 1.45%
1,000,000 41047 bytes -3 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
41356 bytes +309 bytes (+0.75%) +124 bytes
41356 bytes +309 bytes (+0.75%) +124 bytes
41352 bytes +305 bytes (+0.74%) +120 bytes
41287 bytes +240 bytes (+0.58%) +55 bytes
41251 bytes +204 bytes (+0.50%) +19 bytes
41264 bytes +217 bytes (+0.53%) +32 bytes
41245 bytes +198 bytes (+0.48%) +13 bytes
41232 bytes +185 bytes (+0.45%)
41254 bytes +207 bytes (+0.50%) +22 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 30056 bytes -10991 bytes (-26.78%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 32538 bytes -8509 bytes (-20.73%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 34682 bytes -6365 bytes (-15.51%)
Brotli (Wikipedia) brotli brotli -q 11 36208 bytes -4839 bytes (-11.79%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 36787 bytes -4260 bytes (-10.38%)
LZMA2 (Wikipedia) xz xz -9 37408 bytes -3639 bytes (-8.87%)
ZSTD (Wikipedia) zstd zstd -19 39025 bytes -2022 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 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