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:
gzip -6 (default)
  6289 bytes (6.1k)
local copy
unpkg
  6270 bytes (6.1k)
CDN, direct link
gzip -9
  6267 bytes (6.1k)
local copy
7zip -mx=9 -tgzip
  6120 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
  6058 bytes (5.9k)
local copy
Zopfli
  6005 bytes (5.9k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/react/react-15.5.1.min.js

You will automatically get the smallest React 15.5.1 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 265 bytes by using my React 15.5.1 Zopfli version instead of the best available CDN (4.41% smaller than unpkg, 6005 vs. 6270 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 --mls1024 --bsr25 --lazy --ohh

(found April 11, 2017)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 1024  --mls1024
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-15.5.1.min.js --location | md5sum
981fb089ab64a39d58c1d909daa283a0  -
curl --silent --compressed http://minime.stephan-brumme.com/files/react/react-15.5.1.min.zopfli.js.gz | md5sum
981fb089ab64a39d58c1d909daa283a0  -

SHA1:
curl --silent --compressed https://fb.me/react-15.5.1.min.js --location | sha1sum
9ad36382c62bd369ab0b32b8350df1479e4ecb4e  -
curl --silent --compressed http://minime.stephan-brumme.com/files/react/react-15.5.1.min.zopfli.js.gz | sha1sum
9ad36382c62bd369ab0b32b8350df1479e4ecb4e  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
unpkg 6270 bytes 981fb089ab64a39d58c1d909daa283a0 April 11, 2017 @ 11:45

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
6005 bytes -1 byte zopfli --i1000000 --mls1024 --bsr25 --lazy --ohh April 11, 2017 @ 16:17
6006 bytes -1 byte zopfli --i100000 --mls1024 --bsr25 --lazy --ohh April 11, 2017 @ 13:39
6007 bytes -1 byte zopfli --i100000 --mls128 --bsr25 --lazy --ohh April 11, 2017 @ 13:23
6008 bytes -1 byte zopfli --i10000 --mls128 --bsr25 --lazy --ohh April 11, 2017 @ 12:25
6009 bytes -1 byte zopfli --i1000 --mls128 --bsr25 --lazy --ohh April 11, 2017 @ 11:57
6010 bytes -1 byte zopfli --i1000 --mls1024 --bsr22 --lazy --ohh April 11, 2017 @ 11:56
6011 bytes zopfli --i100 --mls1024 --bsr25 --lazy --ohh April 11, 2017 @ 11:48

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

Most recent activity on April 11, 2017 @ 17:29.

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
6109 6109 6109 6106 6106 6106 6106 6106 6106 6106 6106 6104 6110 6109 6109
6111 6111 6108 6105 6105 6105 6106 6106 6106 6106 6106 6103 6103 6105 6105
6108 6107 6109 6105 6109 6106 6106 6106 6106 6106 6106 6104 6103 6033 6033
6109 6107 6109 6109 6109 6025 6106 6106 6106 6100 6106 6028 6110 6101 6101
6109 6109 6109 6109 6110 6103 6106 6101 6106 6106 6106 6104 6102 6101 6101
6109 6109 6107 6107 6105 6105 6106 6101 6106 6106 6106 6104 6104 6071 6031
6101 6110 6109 6037 6106 6027 6106 6106 6106 6100 6106 6104 6103 6103 6103
6109 6109 6109 6104 6108 6106 6008 6106 6106 6106 6106 6022 6109 6101 6101
6110 6110 6102 6109 6106 6028 6106 6106 6106 6101 6106 6104 6102 6101 6101
6111 6109 6109 6109 6108 6025 6106 6106 6106 6106 6106 6104 6102 6104 6104
6111 6110 6109 6107 6105 6026 6106 6101 6106 6106 6106 6104 6104 6069 6018
6109 6109 6109 6109 6105 6026 6106 6106 6106 6101 6106 6104 6104 6101 6101
6109 6110 6109 6109 6106 6105 6106 6106 6012 6106 6106 6104 6102 6026 6026
6109 6109 6109 6109 6108 6024 6106 6106 6106 6106 6106 6104 6103 6101 6101
6104 6106 6106 6105 6106 6106 6106 6106 6106 6101 6106 6104 6109 6101 6101
6103 6103 6109 6107 6109 6105 6007 6106 6012 6106 6106 6104 6109 6105 6103
6109 6109 6031 6106 6108 6026 6106 6101 6106 6101 6106 6104 6104 6101 6109
6109 6109 6109 6109 6105 6105 6106 6101 6106 6101 6106 6104 6102 6101 6033
6110 6110 6109 6109 6109 6007 6008 6106 6011 6007 6106 6104 6110 6101 6101
6109 6109 6109 6109 6105 6031 6106 6106 6106 6100 6106 6026 6103 6109 6105
6110 6109 6109 6104 6108 6012 6007 6106 6007 6005 6106 6104 6104 6104 6104
6109 6109 6100 6101 6106 6105 6106 6106 6106 6101 6106 6104 6102 6104 6104
6103 6111 6009 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 6011 bytes 100%
1,000 6009 bytes -2 bytes 100%
10,000 6008 bytes -1 byte 100%
100,000 6006 bytes -2 bytes 2.90%
1,000,000 6005 bytes -1 byte 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
6102 bytes +97 bytes (+1.62%) +44 bytes
6102 bytes +97 bytes (+1.62%) +44 bytes
6101 bytes +96 bytes (+1.60%) +43 bytes
6064 bytes +59 bytes (+0.98%) +6 bytes
6058 bytes +53 bytes (+0.88%)
6087 bytes +82 bytes (+1.37%) +29 bytes
6112 bytes +107 bytes (+1.78%) +54 bytes
6147 bytes +142 bytes (+2.36%) +89 bytes
6163 bytes +158 bytes (+2.63%) +105 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 -449 bytes (-7.48%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 5579 bytes -426 bytes (-7.09%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 5939 bytes -66 bytes (-1.10%)
LZMA2 (Wikipedia) xz xz -9 5968 bytes -37 bytes (-0.62%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 6061 bytes +56 bytes (+0.93%)
ZSTD (Wikipedia) zstd zstd -19 6079 bytes +74 bytes (+1.23%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 6227 bytes +222 bytes (+3.70%)

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