Choose a version:
21% The original file has 702373 bytes (685.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 148665 bytes (145.2k, 21%).

After GZIP compression these minified files vary in size:
Boot
  52019 bytes (50.8k)
CDN
cdnjs
  44778 bytes (43.7k)
CDN
gzip -6 (default)
  44347 bytes (43.3k)
local copy
cdnhttps
  44332 bytes (43.3k)
CDN
unpkg
  44332 bytes (43.3k)
CDN
jsdelivr
  44266 bytes (43.2k)
CDN
gzip -9
  44172 bytes (43.1k)
local copy
7zip -mx=9 -tgzip
  42715 bytes (41.7k)
local copy
libdeflate -12
  42686 bytes (41.7k)
local copy
kzip -s0 -rn -b7
  42618 bytes (41.6k)
local copy
pigz -11 -n
  42593 bytes (41.6k)
local copy
Zopfli
  42440 bytes (41.4k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/react/react-15.3.1.min.js (or via HTTPS)

You will automatically get the smallest React 15.3.1 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 1826 bytes by using my React 15.3.1 Zopfli version instead of the best available CDN (4.30% smaller than jsdelivr, 42440 vs. 44266 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 --mls2 --bsr17 --lazy --ohh

(found August 25, 2016)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 2  --mls2
block splitting recursion 17  --bsr17
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-15.3.1.min.js --location | md5sum
025fc2741f801b13c3a27c7feef9fb54  -
curl --silent --compressed https://minime.stephan-brumme.com/files/react/react-15.3.1.min.zopfli.js.gz | md5sum
025fc2741f801b13c3a27c7feef9fb54  -

SHA1:
curl --silent --compressed https://fb.me/react-15.3.1.min.js --location | sha1sum
af01780bb8e4ab3ccc6f2ac7172286d4688aa860  -
curl --silent --compressed https://minime.stephan-brumme.com/files/react/react-15.3.1.min.zopfli.js.gz | sha1sum
af01780bb8e4ab3ccc6f2ac7172286d4688aa860  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 52019 bytes 025fc2741f801b13c3a27c7feef9fb54 (invalid)
cdnjs 44778 bytes 025fc2741f801b13c3a27c7feef9fb54 (invalid)
cdnhttps 44332 bytes 025fc2741f801b13c3a27c7feef9fb54 November 22, 2016 @ 16:40
unpkg 44332 bytes 025fc2741f801b13c3a27c7feef9fb54 (invalid)
jsdelivr 44266 bytes 025fc2741f801b13c3a27c7feef9fb54 (invalid)

Note: only the MD5 hashes are shown to keep things simple.

Other Versions

Available React versions at minime.stephan-brumme.com:

16.6.1, 16.6.0,
16.5.2, 16.5.1, 16.5.0,
16.4.2, 16.4.1, 16.4.0,
16.3.2, 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
42440 bytes -5 bytes zopfli --i1000000 --mls2 --bsr17 --lazy --ohh August 25, 2016 @ 15:33
42445 bytes -4 bytes zopfli --i100000 --mls2 --bsr17 --lazy --ohh August 24, 2016 @ 16:32
42449 bytes -4 bytes zopfli --i10000 --mls2 --bsr17 --lazy --ohh August 24, 2016 @ 15:39
42453 bytes -1 byte zopfli --i1000 --mls2 --bsr17 --lazy --ohh August 24, 2016 @ 15:33
42454 bytes -2 bytes zopfli --i1000 --mls64 --bsr21 --lazy --ohh August 24, 2016 @ 15:32
42456 bytes -22 bytes zopfli --i1000 --mls64 --bsr20 --lazy --ohh August 24, 2016 @ 15:32
42478 bytes zopfli --i100 --mls64 --bsr20 --lazy --ohh August 24, 2016 @ 15:30

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
42570 42565 42562 42643 42643 42644 42646 42646 42645 42646 42642 42647 42643 42649 42655
42558 42562 42562 42562 42576 42644 42575 42649 42568 42573 42575 42642 42644 42653 42653
42551 42551 42548 42556 42554 42543 42561 42644 42549 42554 42551 42643 42653 42659 42658
42551 42477 42466 42549 42556 42542 42553 42550 42550 42549 42547 42641 42654 42651 42651
42462 42496 42497 42466 42471 42480 42462 42644 42464 42550 42644 42642 42655 42648 42646
42467 42475 42479 42551 42551 42544 42571 42642 42575 42570 42576 42646 42655 42650 42644
42551 42551 42544 42546 42556 42556 42550 42548 42543 42550 42550 42643 42654 42652 42645
42472 42560 42541 42549 42552 42544 42552 42647 42547 42547 42549 42640 42654 42650 42646
42551 42551 42548 42553 42548 42548 42551 42549 42544 42546 42547 42642 42652 42653 42651
42468 42466 42545 42551 42553 42548 42548 42549 42545 42548 42550 42644 42652 42652 42645
42469 42547 42477 42551 42555 42477 42644 42644 42550 42544 42642 42641 42654 42648 42646
42469 42546 42561 42550 42466 42553 42546 42552 42544 42552 42644 42640 42654 42653 42646
42553 42562 42549 42551 42551 42549 42550 42642 42544 42549 42545 42642 42652 42650 42645
42440 42554 42548 42550 42554 42544 42550 42550 42544 42553 42550 42641 42652 42647 42645
42550 42470 42548 42471 42552 42469 42551 42548 42540 42547 42546 42643 42651 42652 42644
42469 42553 42544 42555 42550 42547 42548 42550 42549 42547 42547 42640 42652 42652 42645
42463 42548 42555 42553 42556 42446 42547 42644 42545 42546 42638 42643 42649 42652 42652
42475 42481 42474 42552 42472 42441 42465 42551 42462 42464 42644 42646 42652 42648 42646
42549 42548 42552 42552 42552 42546 42549 42549 42543 42550 42641 42640 42652 42645 42645
42464 42545 42545 42552 42555 42546 42548 42551 42544 42547 42548 42643 42648 42648 42648
42546 42548 42559 42561 42550 42540 42553 42552 42545 42548 42551 42643 42654 42646 42646
42470 42548 42473 42549 42549 42546 42549 42642 42453 42547 42551 42641 42654 42646 42645
42461 42553 42548 42550 42551 42543 42547 42550 42547 42549 42638 42640 42651 42646 42645

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 42478 bytes 100%
1,000 42453 bytes -25 bytes 100%
10,000 42449 bytes -4 bytes 100%
100,000 42445 bytes -4 bytes 1.16%
1,000,000 42440 bytes -5 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
42735 bytes +295 bytes (+0.70%) +117 bytes
42733 bytes +293 bytes (+0.69%) +115 bytes
42740 bytes +300 bytes (+0.71%) +122 bytes
42678 bytes +238 bytes (+0.56%) +60 bytes
42643 bytes +203 bytes (+0.48%) +25 bytes
42647 bytes +207 bytes (+0.49%) +29 bytes
42644 bytes +204 bytes (+0.48%) +26 bytes
42618 bytes +178 bytes (+0.42%)
42663 bytes +223 bytes (+0.53%) +45 bytes

Non-DEFLATE Algorithms

Archivers based on completely different compression algorithms often produce superior results.
Unfortunately, browsers only support gzip compression at the moment.
However, support for Brotli is constantly growing - but your browser doesn't support it.
Algorithm Program Parameters Size Compared To Best Zopfli
ZPAQ (Wikipedia) zpaq zpaq -method 69 31056 bytes -11384 bytes (-26.82%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 33679 bytes -8761 bytes (-20.64%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 35718 bytes -6722 bytes (-15.84%)
Brotli (Wikipedia) brotli brotli -q 11 37330 bytes -5110 bytes (-12.04%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 38012 bytes -4428 bytes (-10.43%)
LZMA2 (Wikipedia) xz xz -9 38600 bytes -3840 bytes (-9.05%)
Zstandard (Wikipedia) zstd zstd -19 40314 bytes -2126 bytes (-5.01%)

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