Choose a version:
22% The original file has 690515 bytes (674.3k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 153133 bytes (149.5k, 22%).

After GZIP compression these minified files vary in size:
Boot
  53530 bytes (52.3k)
CDN
cdnjs
  45934 bytes (44.9k)
CDN
gzip -6 (default)
  45516 bytes (44.4k)
local copy
cdnhttps
  45472 bytes (44.4k)
CDN
unpkg
  45472 bytes (44.4k)
CDN
jsdelivr
  45403 bytes (44.3k)
CDN
gzip -9
  45305 bytes (44.2k)
local copy
7zip -mx=9 -tgzip
  43890 bytes (42.9k)
local copy
libdeflate -12
  43795 bytes (42.8k)
local copy
kzip -s0 -rn -b7
  43724 bytes (42.7k)
local copy
pigz -11 -n
  43672 bytes (42.6k)
local copy
Zopfli
  43537 bytes (42.5k)
local copy
Zopfli (defluff)
  43534 bytes (42.5k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/react/react-15.2.0.min.js (or via HTTPS)

You will automatically get the smallest React 15.2.0 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 1866 bytes by using my React 15.2.0 Zopfli version instead of the best available CDN (4.29% smaller than jsdelivr, 43537 vs. 45403 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 --mls16 --bsr21 --lazy --ohh

(found July 5, 2016)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 16  --mls16
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 3 more bytes (43534 bytes).

Verify file integrity

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

MD5:
curl --silent --compressed https://fb.me/react-15.2.0.min.js --location | md5sum
b5c43dd923379d5256128fa97988a3d1  -
curl --silent --compressed https://minime.stephan-brumme.com/files/react/react-15.2.0.min.zopfli.js.gz | md5sum
b5c43dd923379d5256128fa97988a3d1  -

SHA1:
curl --silent --compressed https://fb.me/react-15.2.0.min.js --location | sha1sum
e867c4c4ca666810d7b141162c24c1d460f5802f  -
curl --silent --compressed https://minime.stephan-brumme.com/files/react/react-15.2.0.min.zopfli.js.gz | sha1sum
e867c4c4ca666810d7b141162c24c1d460f5802f  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 53530 bytes b5c43dd923379d5256128fa97988a3d1 July 3, 2016 @ 05:50
cdnjs 45934 bytes b5c43dd923379d5256128fa97988a3d1 July 1, 2016 @ 21:16
cdnhttps 45472 bytes b5c43dd923379d5256128fa97988a3d1 November 22, 2016 @ 16:40
unpkg 45472 bytes b5c43dd923379d5256128fa97988a3d1 July 11, 2016 @ 15:32
jsdelivr 45403 bytes b5c43dd923379d5256128fa97988a3d1 July 1, 2016 @ 21:06

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
43537 bytes -3 bytes zopfli --i1000000 --mls16 --bsr21 --lazy --ohh July 5, 2016 @ 01:33
43540 bytes -3 bytes zopfli --i100000 --mls16 --bsr21 --lazy --ohh July 4, 2016 @ 12:08
43543 bytes -3 bytes zopfli --i10000 --mls16 --bsr21 --lazy --ohh July 4, 2016 @ 11:14
43546 bytes -4 bytes zopfli --i1000 --mls16 --bsr15 --lazy --ohh July 4, 2016 @ 11:08
43550 bytes -16 bytes zopfli --i1000 --mls16 --bsr21 --lazy --ohh July 4, 2016 @ 11:07
43566 bytes zopfli --i100 --mls16 --bsr21 --lazy --ohh July 4, 2016 @ 11:06

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
43737 43737 43737 43738 43738 43743 43743 43744 43746 43747 43749 43743 43753 43753 43749
43743 43740 43746 43592 43743 43574 43669 43659 43663 43659 43661 43738 43751 43747 43757
43740 43745 43748 43646 43644 43644 43645 43743 43644 43743 43635 43738 43747 43747 43754
43737 43737 43737 43637 43640 43637 43636 43641 43639 43638 43636 43737 43748 43747 43749
43740 43740 43745 43563 43639 43542 43558 43581 43556 43554 43634 43737 43749 43752 43747
43740 43740 43748 43638 43640 43555 43560 43555 43636 43637 43637 43737 43750 43752 43751
43744 43740 43748 43639 43639 43638 43643 43641 43637 43635 43636 43737 43750 43752 43751
43737 43737 43737 43638 43635 43638 43636 43638 43638 43636 43635 43738 43752 43752 43751
43740 43740 43748 43636 43638 43635 43639 43639 43637 43635 43641 43737 43750 43754 43753
43745 43741 43745 43644 43636 43637 43637 43639 43640 43636 43639 43737 43750 43748 43750
43744 43737 43748 43555 43639 43556 43555 43552 43637 43742 43635 43737 43752 43753 43751
43745 43740 43747 43541 43553 43646 43641 43640 43642 43553 43740 43737 43750 43754 43749
43745 43740 43748 43638 43638 43639 43638 43640 43638 43634 43638 43737 43748 43751 43749
43745 43740 43748 43641 43635 43636 43641 43634 43638 43635 43639 43738 43747 43755 43751
43740 43740 43746 43639 43640 43560 43639 43558 43635 43555 43636 43737 43754 43754 43753
43740 43740 43748 43637 43640 43639 43641 43639 43638 43636 43639 43737 43750 43754 43753
43740 43740 43748 43640 43634 43636 43639 43636 43640 43633 43636 43737 43751 43752 43751
43740 43740 43748 43537 43554 43556 43559 43564 43554 43562 43740 43737 43748 43752 43747
43745 43740 43745 43637 43637 43637 43643 43640 43561 43553 43638 43737 43749 43748 43749
43744 43741 43746 43639 43638 43643 43637 43639 43638 43632 43637 43737 43750 43752 43748
43740 43740 43746 43639 43637 43636 43642 43638 43638 43637 43637 43737 43750 43747 43748
43745 43740 43747 43639 43641 43634 43637 43635 43638 43638 43639 43738 43750 43752 43753
43740 43740 43748 43639 43641 43557 43554 43553 43554 43557 43642 43738 43752 43752 43751

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 43566 bytes 100%
1,000 43546 bytes -20 bytes 100%
10,000 43543 bytes -3 bytes 100%
100,000 43540 bytes -3 bytes 0.87%
1,000,000 43537 bytes -3 bytes 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
43831 bytes +294 bytes (+0.68%) +107 bytes
43831 bytes +294 bytes (+0.68%) +107 bytes
43844 bytes +307 bytes (+0.71%) +120 bytes
43785 bytes +248 bytes (+0.57%) +61 bytes
43747 bytes +210 bytes (+0.48%) +23 bytes
43760 bytes +223 bytes (+0.51%) +36 bytes
43734 bytes +197 bytes (+0.45%) +10 bytes
43724 bytes +187 bytes (+0.43%)
43736 bytes +199 bytes (+0.46%) +12 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 31800 bytes -11737 bytes (-26.96%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 34393 bytes -9144 bytes (-21.00%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 36857 bytes -6680 bytes (-15.34%)
Brotli (Wikipedia) brotli brotli -q 11 38319 bytes -5218 bytes (-11.99%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 38856 bytes -4681 bytes (-10.75%)
LZMA2 (Wikipedia) xz xz -9 39588 bytes -3949 bytes (-9.07%)
Zstandard (Wikipedia) zstd zstd -19 41318 bytes -2219 bytes (-5.10%)

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