Choose a version:
16% The original file has 145032 bytes (141.6k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 23040 bytes (22.5k, 16%).

After GZIP compression these minified files vary in size:
Boot
  8941 bytes (8.7k)
CDN, direct link
cdnjs
  7971 bytes (7.8k)
CDN, direct link
gzip -6 (default)
  7957 bytes (7.8k)
local copy
unpkg
  7945 bytes (7.8k)
CDN, direct link
gzip -9
  7935 bytes (7.7k)
local copy
libdeflate -12
  7747 bytes (7.6k)
local copy
7zip -mx=9 -tgzip
  7746 bytes (7.6k)
local copy
kzip -s0 -rn -b6
  7734 bytes (7.6k)
local copy
pigz -11 -n
  7633 bytes (7.5k)
local copy
Zopfli
  7621 bytes (7.4k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/react/react-15.6.1.min.js

You will automatically get the smallest React 15.6.1 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 324 bytes by using my React 15.6.1 Zopfli version instead of the best available CDN (4.25% smaller than unpkg, 7621 vs. 7945 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 --i10000000 --mb8 --mls256 --bsr9 --lazy --ohh

(found June 17, 2017)
Description Value Parameter
iterations 10000000  --i10000000
maximum blocks 8  --mb8
maximum length score 256  --mls256
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-15.6.1.min.js --location | md5sum
6edad0b09347612195c91e053c8aa038  -
curl --silent --compressed http://minime.stephan-brumme.com/files/react/react-15.6.1.min.zopfli.js.gz | md5sum
6edad0b09347612195c91e053c8aa038  -

SHA1:
curl --silent --compressed https://fb.me/react-15.6.1.min.js --location | sha1sum
a39545e8f952278001ebe002d7dbd02fe1bddb67  -
curl --silent --compressed http://minime.stephan-brumme.com/files/react/react-15.6.1.min.zopfli.js.gz | sha1sum
a39545e8f952278001ebe002d7dbd02fe1bddb67  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 8941 bytes 6edad0b09347612195c91e053c8aa038 June 18, 2017 @ 16:01
cdnjs 7971 bytes 6edad0b09347612195c91e053c8aa038 (invalid)
unpkg 7945 bytes 6edad0b09347612195c91e053c8aa038 June 16, 2017 @ 10:12

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
7621 bytes -1 byte zopfli --i10000000 --mls256 --bsr9 --lazy --ohh June 17, 2017 @ 01:47
7622 bytes -1 byte zopfli --i100000 --mls256 --bsr9 --lazy --ohh June 16, 2017 @ 11:08
7623 bytes -1 byte zopfli --i10000 --mls256 --bsr9 --lazy --ohh June 16, 2017 @ 10:19
7624 bytes -1 byte zopfli --i1000 --mls128 --bsr17 --lazy --ohh June 16, 2017 @ 10:16
7625 bytes -2 bytes zopfli --i1000 --mls64 --bsr8 --lazy --ohh June 16, 2017 @ 10:16
7627 bytes zopfli --i100 --mls256 --bsr9 --lazy --ohh June 16, 2017 @ 10:13

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

Most recent activity on June 18, 2017 @ 16:01.

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 10,000,000.
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
7672 7672 7673 7673 7674 7675 7673 7672 7675 7673 7644 7674 7675 7672 7672
7629 7628 7631 7639 7626 7634 7651 7656 7637 7631 7637 7637 7627 7632 7626
7649 7649 7629 7631 7628 7627 7629 7622 7627 7627 7623 7627 7641 7625 7625
7632 7631 7633 7629 7633 7626 7624 7624 7624 7627 7623 7624 7625 7629 7626
7631 7631 7629 7631 7623 7624 7626 7625 7624 7624 7626 7624 7625 7628 7625
7629 7629 7629 7629 7623 7629 7626 7621 7624 7623 7625 7624 7625 7628 7626
7632 7632 7628 7629 7624 7625 7626 7622 7626 7625 7625 7624 7626 7629 7628
7633 7632 7629 7629 7641 7641 7640 7637 7639 7625 7640 7627 7625 7629 7629
7633 7633 7652 7631 7623 7626 7626 7625 7625 7623 7627 7627 7627 7628 7625
7629 7629 7628 7631 7623 7629 7626 7625 7624 7624 7624 7622 7626 7626 7627
7624 7635 7627 7629 7625 7625 7625 7626 7625 7627 7626 7624 7626 7626 7625
7630 7631 7629 7634 7624 7626 7625 7622 7625 7624 7625 7627 7627 7628 7626
7634 7629 7627 7631 7627 7633 7626 7624 7629 7633 7625 7624 7627 7627 7627
7636 7631 7631 7629 7622 7629 7624 7625 7624 7624 7623 7627 7626 7626 7626
7633 7633 7629 7629 7623 7626 7626 7626 7624 7624 7623 7624 7626 7626 7626
7629 7629 7622 7629 7624 7625 7624 7622 7626 7624 7626 7626 7627 7627 7627
7632 7632 7629 7629 7624 7622 7627 7622 7624 7624 7623 7627 7627 7629 7627
7635 7631 7629 7623 7627 7624 7624 7628 7628 7625 7624 7627 7625 7627 7625
7629 7632 7625 7625 7626 7622 7626 7624 7625 7625 7623 7628 7628 7628 7628
7629 7635 7629 7627 7623 7628 7629 7623 7626 7627 7625 7623 7627 7629 7629
7631 7631 7628 7625 7623 7626 7628 7625 7624 7624 7623 7623 7627 7628 7628
7625 7630 7626 7626 7625 7630 7625 7623 7624 7626 7624 7625 7626 7629 7629
7634 7635 7628 7627 7623 7632 7626 7625 7624 7627 7623 7633 7625 7627 7627

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 7627 bytes 100%
1,000 7624 bytes -3 bytes 100%
10,000 7623 bytes -1 byte 100%
100,000 7622 bytes -1 byte 9.86%
1,000,000 7622 bytes 0.29%
10,000,000 7621 bytes -1 byte 0.29%

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
7741 bytes +120 bytes (+1.57%) +7 bytes
7741 bytes +120 bytes (+1.57%) +7 bytes
7741 bytes +120 bytes (+1.57%) +7 bytes
7773 bytes +152 bytes (+1.99%) +39 bytes
7800 bytes +179 bytes (+2.35%) +66 bytes
7798 bytes +177 bytes (+2.32%) +64 bytes
7734 bytes +113 bytes (+1.48%)
7770 bytes +149 bytes (+1.96%) +36 bytes
7787 bytes +166 bytes (+2.18%) +53 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 6976 bytes -645 bytes (-8.46%)
Brotli (Wikipedia) brotli brotli -q 11 7000 bytes -621 bytes (-8.15%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 7410 bytes -211 bytes (-2.77%)
LZMA2 (Wikipedia) xz xz -9 7552 bytes -69 bytes (-0.91%)
ZSTD (Wikipedia) zstd zstd -19 7681 bytes +60 bytes (+0.79%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 7699 bytes +78 bytes (+1.02%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 7852 bytes +231 bytes (+3.03%)

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