Choose a version:
13% The original file has 136018 bytes (132.8k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 17488 bytes (17.1k, 13%).

After GZIP compression these minified files vary in size:
Boot
  6922 bytes (6.8k)
CDN
cdnjs
  6291 bytes (6.1k)
CDN
gzip -6 (default)
  6290 bytes (6.1k)
local copy
unpkg
  6270 bytes (6.1k)
CDN
gzip -9
  6267 bytes (6.1k)
local copy
jsdelivr
  6267 bytes (6.1k)
CDN
7zip -mx=9 -tgzip
  6118 bytes (6.0k)
local copy
libdeflate -12
  6107 bytes (6.0k)
local copy
pigz -11 -n
  6107 bytes (6.0k)
local copy
kzip -s0 -rn -b4
  6057 bytes (5.9k)
local copy
Zopfli
  6006 bytes (5.9k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/react/react-15.5.0.min.js

You will automatically get the smallest React 15.5.0 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 261 bytes by using my React 15.5.0 Zopfli version instead of the best available CDN (4.35% smaller than jsdelivr, 6006 vs. 6267 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 --mls512 --bsr25 --lazy --ohh

(found April 11, 2017)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 512  --mls512
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-15.5.0.min.js --location | md5sum
f6576ee1e958eda7d0ca88abd885ec47  -
curl --silent --compressed http://minime.stephan-brumme.com/files/react/react-15.5.0.min.zopfli.js.gz | md5sum
f6576ee1e958eda7d0ca88abd885ec47  -

SHA1:
curl --silent --compressed https://fb.me/react-15.5.0.min.js --location | sha1sum
8b17c7979240e1396d42cb183c7fb3d2b74c2ff0  -
curl --silent --compressed http://minime.stephan-brumme.com/files/react/react-15.5.0.min.zopfli.js.gz | sha1sum
8b17c7979240e1396d42cb183c7fb3d2b74c2ff0  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 6922 bytes f6576ee1e958eda7d0ca88abd885ec47 April 12, 2017 @ 22:58
cdnjs 6291 bytes f6576ee1e958eda7d0ca88abd885ec47 (invalid)
unpkg 6270 bytes f6576ee1e958eda7d0ca88abd885ec47 April 10, 2017 @ 12:19
jsdelivr 6267 bytes f6576ee1e958eda7d0ca88abd885ec47 (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
6006 bytes -1 byte zopfli --i1000000 --mls512 --bsr25 --lazy --ohh April 11, 2017 @ 18:53
6007 bytes -1 byte zopfli --i100000 --mls512 --bsr25 --lazy --ohh April 11, 2017 @ 13:23
6008 bytes -1 byte zopfli --i10000 --mls512 --bsr25 --lazy --ohh April 11, 2017 @ 11:01
6009 bytes -1 byte zopfli --i1000 --mls512 --bsr25 --lazy --ohh April 11, 2017 @ 10:47
6010 bytes -1 byte zopfli --i1000 --mls128 --bsr19 --lazy --ohh April 11, 2017 @ 10:46
6011 bytes -1 byte zopfli --i1000 --mls128 --bsr11 --lazy --ohh April 11, 2017 @ 10:46
6012 bytes zopfli --i100 --mls64 --bsr22 --lazy --ohh April 10, 2017 @ 12:28

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

Most recent activity on April 12, 2017 @ 22:58.

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
6110 6110 6109 6106 6106 6106 6106 6106 6106 6106 6106 6104 6110 6109 6109
6111 6111 6107 6105 6106 6027 6106 6106 6106 6106 6106 6103 6103 6106 6106
6110 6107 6109 6105 6109 6106 6106 6106 6106 6106 6106 6104 6103 6033 6032
6109 6107 6109 6109 6109 6025 6106 6106 6106 6100 6106 6028 6110 6101 6101
6109 6109 6109 6109 6110 6108 6106 6106 6106 6106 6106 6104 6102 6101 6101
6109 6109 6106 6107 6106 6107 6106 6101 6106 6106 6106 6104 6104 6071 6031
6101 6110 6109 6036 6106 6028 6106 6101 6106 6100 6106 6104 6103 6104 6104
6109 6109 6109 6105 6108 6106 6009 6106 6106 6106 6106 6022 6104 6101 6101
6110 6110 6109 6109 6106 6028 6106 6106 6106 6101 6106 6104 6102 6101 6101
6111 6109 6109 6105 6108 6027 6106 6106 6106 6106 6106 6104 6102 6104 6104
6111 6110 6104 6107 6106 6026 6106 6103 6106 6106 6106 6104 6104 6103 6019
6109 6108 6109 6109 6110 6026 6106 6106 6106 6101 6106 6104 6104 6101 6101
6111 6110 6109 6109 6106 6102 6106 6106 6013 6106 6106 6104 6102 6064 6027
6109 6109 6109 6105 6109 6024 6106 6102 6106 6106 6106 6104 6103 6101 6101
6104 6106 6109 6105 6106 6106 6106 6103 6106 6101 6106 6104 6109 6101 6019
6103 6103 6109 6107 6109 6011 6006 6106 6012 6106 6106 6104 6110 6106 6103
6109 6109 6031 6109 6109 6026 6106 6101 6106 6101 6106 6104 6104 6101 6109
6109 6109 6109 6109 6106 6101 6106 6101 6106 6101 6106 6104 6103 6101 6014
6110 6110 6109 6109 6109 6007 6009 6106 6011 6106 6106 6104 6104 6101 6101
6109 6107 6109 6109 6109 6031 6106 6101 6106 6100 6106 6026 6103 6109 6106
6110 6109 6109 6105 6108 6012 6006 6106 6006 6106 6106 6104 6104 6104 6104
6109 6109 6101 6101 6106 6102 6106 6102 6106 6101 6106 6104 6104 6104 6104
6103 6111 6010 6109 6109 6107 6009 6106 6106 6100 6106 6103 6104 6103 6021

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 6012 bytes 100%
1,000 6009 bytes -3 bytes 100%
10,000 6008 bytes -1 byte 100%
100,000 6007 bytes -1 byte 2.32%
1,000,000 6006 bytes -1 byte 1.16%
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
6102 bytes +96 bytes (+1.60%) +45 bytes
6103 bytes +97 bytes (+1.62%) +46 bytes
6099 bytes +93 bytes (+1.55%) +42 bytes
6062 bytes +56 bytes (+0.93%) +5 bytes
6057 bytes +51 bytes (+0.85%)
6084 bytes +78 bytes (+1.30%) +27 bytes
6110 bytes +104 bytes (+1.73%) +53 bytes
6142 bytes +136 bytes (+2.26%) +85 bytes
6160 bytes +154 bytes (+2.56%) +103 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
Brotli (Wikipedia) brotli brotli -q 11 5556 bytes -450 bytes (-7.49%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 5579 bytes -427 bytes (-7.11%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 5939 bytes -67 bytes (-1.12%)
LZMA2 (Wikipedia) xz xz -9 5976 bytes -30 bytes (-0.50%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 6062 bytes +56 bytes (+0.93%)
ZSTD (Wikipedia) zstd zstd -19 6079 bytes +73 bytes (+1.22%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 6227 bytes +221 bytes (+3.68%)

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