Choose a version:
44% The original file has 126044 bytes (123.1k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 55775 bytes (54.5k, 44%).

After GZIP compression these minified files vary in size:
cdnjs
  14722 bytes (14.4k)
CDN
gzip -6 (default)
  14573 bytes (14.2k)
local copy
MaxCDN
  14550 bytes (14.2k)
CDN
gzip -9
  14509 bytes (14.2k)
local copy
unpkg
  14486 bytes (14.1k)
CDN
7zip -mx=9 -tgzip
  14038 bytes (13.7k)
local copy
libdeflate -12
  14030 bytes (13.7k)
local copy
kzip -s0 -rn -b2
  14024 bytes (13.7k)
local copy
pigz -11 -n
  13991 bytes (13.7k)
local copy
Zopfli
  13983 bytes (13.7k)
local copy
Zopfli (defluff)
  13982 bytes (13.7k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.2.1.min.js (or via HTTPS)

You will automatically get the smallest Bootstrap 4.2.1 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 503 bytes by using my Bootstrap 4.2.1 Zopfli version instead of the best available CDN (3.60% smaller than unpkg, 13983 vs. 14486 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 --mls256 --bsr25 --lazy --ohh

(found January 7, 2019)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 256  --mls256
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

Even Smaller Files Thanks To Defluff

Zopfli's output can be further optimized by the defluff tool.
In this particular case, defluff saves 1 more byte (13982 bytes).

Verify file integrity

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

MD5:
curl --silent --compressed https://raw.githubusercontent.com/twbs/bootstrap/v4.2.1/dist/js/bootstrap.min.js --location | md5sum
6895e8cd60b62646ce12426015888f58  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.2.1.min.zopfli.js.gz | md5sum
6895e8cd60b62646ce12426015888f58  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/twbs/bootstrap/v4.2.1/dist/js/bootstrap.min.js --location | sha1sum
de908c9ed184d74eb525fa7a30449b67fc3a1c14  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.2.1.min.zopfli.js.gz | sha1sum
de908c9ed184d74eb525fa7a30449b67fc3a1c14  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
cdnjs 14722 bytes 6895e8cd60b62646ce12426015888f58 (invalid)
MaxCDN 14550 bytes 6895e8cd60b62646ce12426015888f58 (invalid)
unpkg 14486 bytes 6895e8cd60b62646ce12426015888f58 (invalid)

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

Other Versions

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

4.2.1,
4.1.3, 4.1.2, 4.1.1, 4.1.0,
4.0.0,
3.4.0,
3.3.7, 3.3.6, 3.3.5, 3.3.4, 3.3.2, 3.3.1, 3.3.0,
3.2.0,
3.1.1, 3.1.0,
3.0.3, 3.0.2, 3.0.1, 3.0.0

The project site contains an overview how well these versions were compressed.
Other interesting projects are AngularJS, BackboneJS, D3, Dojo, Ember, jQuery, Knockout, lodash, React, Socket.IO, ThreeJS, UnderscoreJS and Vue.

Changelog

Best Zopfli parameters so far:
Size Improvement Parameters Found
13983 bytes -1 byte zopfli --i1000000 --mls256 --bsr25 --lazy --ohh January 7, 2019 @ 12:14
13984 bytes -1 byte zopfli --i100000 --mls256 --bsr25 --lazy --ohh January 4, 2019 @ 11:05
13985 bytes -1 byte zopfli --i10000 --mls256 --bsr25 --lazy --ohh January 4, 2019 @ 10:02
13986 bytes -1 byte zopfli --i1000 --mls32 --bsr20 --lazy --ohh January 4, 2019 @ 09:49
13987 bytes -1 byte zopfli --i1000 --mls2 --bsr15 --lazy --ohh January 4, 2019 @ 09:48
13988 bytes -1 byte zopfli --i1000 --mls2 --bsr10 --lazy --ohh January 4, 2019 @ 09:47
13989 bytes zopfli --i100 --mls128 --bsr10 --lazy --ohh January 3, 2019 @ 16:44

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

Most recent activity on January 7, 2019 @ 16:45.

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
13988 13987 13987 13987 13988 13986 13984 13985 13988 13987 13989 13988 13988 13986 13987
13986 13986 13988 13987 13987 13987 13986 13986 13986 13986 13986 13987 13998 13987 13985
13987 13987 13987 13988 13987 13987 13988 13987 13987 13987 13986 13986 13988 13986 13987
13986 13986 13987 13988 13986 13984 13984 13986 13985 13985 13987 13987 13990 13987 13987
13986 13986 13987 13987 13986 13987 13984 13986 13985 13984 13986 13985 13989 13985 13987
13986 13986 13987 13984 13987 13984 13984 13984 13987 13987 13987 13987 13987 13987 13986
13986 13986 13989 13986 13986 13985 13986 13986 13986 13986 13987 13988 13987 13988 13987
13987 13987 13987 13984 13986 13985 13986 13986 13984 13986 13988 13985 13987 13988 13996
13986 13986 13987 13984 13986 13984 13984 13986 13986 13984 13987 13985 13999 13987 13987
13986 13986 13987 13988 13986 13984 13984 13986 13987 13984 13986 13986 13990 13988 13987
13986 13986 13987 13986 13986 13986 13984 13985 13985 13985 13985 13986 13989 13987 13987
13986 13986 13987 13988 13986 13986 13984 13986 13985 13985 13986 13986 13987 13987 13987
13987 13987 13988 13987 13987 13985 13984 13986 13985 13987 13987 13987 13988 13987 13996
13986 13986 13986 13986 13986 13986 13984 13985 13986 13986 13988 13987 13987 13987 13996
13986 13986 13989 13985 13986 13986 13986 13984 13987 13985 13987 13987 13986 13987 13987
13987 13986 13986 13986 13986 13987 13986 13986 13986 13987 13986 13986 13987 13986 13987
13986 13986 13989 13986 13986 13986 13984 13986 13985 13984 13984 13986 13987 13988 13988
13986 13986 13986 13984 13986 13987 13986 13986 13987 13985 13986 13986 13990 13986 13987
13986 13986 13986 13987 13986 13985 13986 13986 13984 13984 13987 13987 13989 13986 13987
13986 13986 13988 13987 13986 13986 13986 13986 13987 13984 13987 13986 13990 13988 13987
13986 13986 13988 13987 13986 13986 13984 13983 13985 13985 13987 13985 13987 13987 13987
13986 13986 13988 13984 13986 13986 13984 13986 13985 13987 13986 13987 13987 13986 13987
13986 13986 13987 13987 13986 13987 13986 13987 13987 13986 13987 13985 13987 13986 13987

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 13989 bytes 100%
1,000 13986 bytes -3 bytes 100%
10,000 13985 bytes -1 byte 100%
100,000 13984 bytes -1 byte 18.55%
1,000,000 13983 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
14058 bytes +75 bytes (+0.54%) +34 bytes
14058 bytes +75 bytes (+0.54%) +34 bytes
14024 bytes +41 bytes (+0.29%)
14049 bytes +66 bytes (+0.47%) +25 bytes
14056 bytes +73 bytes (+0.52%) +32 bytes
14090 bytes +107 bytes (+0.77%) +66 bytes
14123 bytes +140 bytes (+1.00%) +99 bytes
14162 bytes +179 bytes (+1.28%) +138 bytes
14163 bytes +180 bytes (+1.29%) +139 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
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 11645 bytes -2338 bytes (-16.72%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12035 bytes -1948 bytes (-13.93%)
Brotli (Wikipedia) brotli brotli -q 11 12751 bytes -1232 bytes (-8.81%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 13477 bytes -506 bytes (-3.62%)
LZMA2 (Wikipedia) xz xz -9 13520 bytes -463 bytes (-3.31%)
Zstandard (Wikipedia) zstd zstd -19 13700 bytes -283 bytes (-2.02%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 13881 bytes -102 bytes (-0.73%)

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 2019.
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