Choose a version:
21% The original file has 701412 bytes (685.0k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 148805 bytes (145.3k, 21%).

After GZIP compression these minified files vary in size:
Boot
  52088 bytes (50.9k)
CDN, direct link
cdnjs
  44823 bytes (43.8k)
CDN, direct link
gzip -6 (default)
  44397 bytes (43.4k)
local copy
cdnhttps
  44374 bytes (43.3k)
CDN, direct link
unpkg
  44374 bytes (43.3k)
CDN, direct link
jsdelivr
  44314 bytes (43.3k)
CDN, direct link
gzip -9
  44216 bytes (43.2k)
local copy
7zip -mx=9 -tgzip
  42749 bytes (41.7k)
local copy
libdeflate -12
  42736 bytes (41.7k)
local copy
kzip -s0 -rn -b7
  42656 bytes (41.7k)
local copy
pigz -11 -n
  42634 bytes (41.6k)
local copy
Zopfli
  42492 bytes (41.5k)
local copy
Zopfli (defluff)
  42490 bytes (41.5k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/react/react-15.3.2.min.js

You will automatically get the smallest React 15.3.2 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 1822 bytes by using my React 15.3.2 Zopfli version instead of the best available CDN (4.29% smaller than jsdelivr, 42492 vs. 44314 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 --bsr21 --lazy --ohh

(found September 21, 2016)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 32  --mls32
block splitting recursion 21  --bsr21
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 (42490 bytes).

Verify file integrity

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

MD5:
curl --silent --compressed https://fb.me/react-15.3.2.min.js --location | md5sum
cbf013a1454179d9d916ff1296872b9a  -
curl --silent --compressed http://minime.stephan-brumme.com/files/react/react-15.3.2.min.zopfli.js.gz | md5sum
cbf013a1454179d9d916ff1296872b9a  -

SHA1:
curl --silent --compressed https://fb.me/react-15.3.2.min.js --location | sha1sum
7cb0f056e2dceb9f5a4939252f1e0f1d7d90e7c0  -
curl --silent --compressed http://minime.stephan-brumme.com/files/react/react-15.3.2.min.zopfli.js.gz | sha1sum
7cb0f056e2dceb9f5a4939252f1e0f1d7d90e7c0  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 52088 bytes cbf013a1454179d9d916ff1296872b9a September 21, 2016 @ 17:22
cdnjs 44823 bytes cbf013a1454179d9d916ff1296872b9a (invalid)
cdnhttps 44374 bytes cbf013a1454179d9d916ff1296872b9a November 22, 2016 @ 16:40
unpkg 44374 bytes cbf013a1454179d9d916ff1296872b9a September 20, 2016 @ 00:21
jsdelivr 44314 bytes cbf013a1454179d9d916ff1296872b9a (invalid)

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
42492 bytes -6 bytes zopfli --i1000000 --mls32 --bsr21 --lazy --ohh September 21, 2016 @ 02:05
42498 bytes -4 bytes zopfli --i100000 --mls32 --bsr21 --lazy --ohh September 20, 2016 @ 10:48
42502 bytes -7 bytes zopfli --i10000 --mls32 --bsr21 --lazy --ohh September 20, 2016 @ 09:48
42509 bytes -15 bytes zopfli --i1000 --mls32 --bsr21 --lazy --ohh September 20, 2016 @ 09:42
42524 bytes zopfli --i100 --mls32 --bsr21 --lazy --ohh September 20, 2016 @ 00:40

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
42613 42608 42582 42684 42684 42684 42687 42685 42686 42689 42688 42690 42686 42692 42700
42537 42604 42603 42532 42615 42685 42620 42691 42607 42613 42605 42687 42692 42698 42689
42598 42592 42601 42595 42591 42594 42690 42692 42591 42598 42590 42682 42692 42696 42697
42507 42514 42521 42596 42591 42597 42594 42689 42587 42586 42593 42684 42693 42694 42687
42512 42513 42517 42516 42511 42688 42687 42690 42503 42509 42589 42688 42690 42689 42693
42510 42509 42520 42594 42593 42689 42591 42689 42592 42588 42613 42691 42690 42692 42691
42595 42586 42591 42592 42597 42595 42593 42592 42591 42593 42594 42684 42691 42696 42690
42504 42527 42512 42592 42590 42588 42595 42690 42586 42586 42590 42686 42692 42691 42610
42592 42594 42517 42592 42591 42596 42593 42691 42590 42585 42590 42686 42694 42697 42694
42512 42591 42598 42590 42594 42595 42592 42595 42591 42586 42591 42682 42693 42694 42688
42513 42586 42598 42594 42494 42689 42689 42689 42591 42587 42590 42682 42691 42698 42689
42590 42595 42596 42593 42598 42689 42686 42689 42590 42593 42685 42685 42695 42698 42689
42590 42596 42603 42593 42593 42592 42594 42690 42587 42588 42592 42686 42694 42689 42694
42509 42521 42594 42593 42591 42593 42594 42690 42591 42587 42591 42682 42694 42699 42691
42512 42510 42515 42512 42593 42689 42593 42690 42588 42587 42590 42686 42694 42698 42692
42510 42587 42596 42592 42598 42594 42597 42592 42594 42586 42589 42684 42693 42689 42693
42502 42508 42597 42594 42515 42688 42690 42690 42592 42590 42590 42682 42689 42688 42692
42507 42518 42530 42596 42492 42688 42690 42690 42511 42502 42685 42685 42692 42690 42688
42587 42589 42594 42594 42592 42594 42592 42686 42590 42592 42686 42690 42692 42690 42693
42499 42513 42596 42595 42591 42597 42593 42689 42590 42588 42589 42686 42694 42688 42693
42512 42593 42597 42612 42592 42596 42593 42687 42591 42589 42592 42682 42691 42690 42689
42511 42515 42596 42512 42590 42595 42592 42691 42504 42586 42590 42686 42694 42697 42689
42511 42595 42599 42592 42589 42594 42595 42593 42591 42591 42594 42686 42694 42699 42692

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 42524 bytes 100%
1,000 42509 bytes -15 bytes 100%
10,000 42502 bytes -7 bytes 100%
100,000 42498 bytes -4 bytes 2.32%
1,000,000 42492 bytes -6 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
42778 bytes +286 bytes (+0.67%) +122 bytes
42778 bytes +286 bytes (+0.67%) +122 bytes
42782 bytes +290 bytes (+0.68%) +126 bytes
42719 bytes +227 bytes (+0.53%) +63 bytes
42685 bytes +193 bytes (+0.45%) +29 bytes
42695 bytes +203 bytes (+0.48%) +39 bytes
42686 bytes +194 bytes (+0.46%) +30 bytes
42656 bytes +164 bytes (+0.39%)
42698 bytes +206 bytes (+0.48%) +42 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 31097 bytes -11395 bytes (-26.82%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 33721 bytes -8771 bytes (-20.64%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 35751 bytes -6741 bytes (-15.86%)
Brotli (Wikipedia) brotli brotli -q 11 37477 bytes -5015 bytes (-11.80%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 38009 bytes -4483 bytes (-10.55%)
LZMA2 (Wikipedia) xz xz -9 38624 bytes -3868 bytes (-9.10%)
ZSTD (Wikipedia) zstd zstd -19 40366 bytes -2126 bytes (-5.00%)

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