Choose a version:
44% The original file has 131637 bytes (128.6k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 58072 bytes (56.7k, 44%).

After GZIP compression these minified files vary in size:
unpkg
  19368 bytes (18.9k)
CDN
cdnjs
  15613 bytes (15.2k)
CDN
gzip -6 (default)
  15457 bytes (15.1k)
local copy
Microsoft
  15435 bytes (15.1k)
CDN
MaxCDN
  15434 bytes (15.1k)
CDN
gzip -9
  15394 bytes (15.0k)
local copy
libdeflate -12
  14932 bytes (14.6k)
local copy
7zip -mx=9 -tgzip
  14911 bytes (14.6k)
local copy
kzip -s0 -rn -b2
  14888 bytes (14.5k)
local copy
pigz -11 -n
  14859 bytes (14.5k)
local copy
Zopfli
  14848 bytes (14.5k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.3.1.min.js (or via HTTPS)

You will automatically get the smallest Bootstrap 4.3.1 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 586 bytes by using my Bootstrap 4.3.1 Zopfli version instead of the best available CDN (3.95% smaller than MaxCDN, 14848 vs. 15434 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 --i100000 --mb8 --mls128 --bsr11 --lazy --ohh

(found February 15, 2019)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 128  --mls128
block splitting recursion 11  --bsr11
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://raw.githubusercontent.com/twbs/bootstrap/v4.3.1/dist/js/bootstrap.min.js --location | md5sum
e1d98d47689e00f8ecbc5d9f61bdb42e  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.3.1.min.zopfli.js.gz | md5sum
e1d98d47689e00f8ecbc5d9f61bdb42e  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/twbs/bootstrap/v4.3.1/dist/js/bootstrap.min.js --location | sha1sum
6778fed3cf095a318141a31f455c8f4663885bde  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.3.1.min.zopfli.js.gz | sha1sum
6778fed3cf095a318141a31f455c8f4663885bde  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
unpkg 19368 bytes e1d98d47689e00f8ecbc5d9f61bdb42e (invalid)
cdnjs 15613 bytes e1d98d47689e00f8ecbc5d9f61bdb42e February 15, 2019 @ 19:45
Microsoft 15435 bytes e1d98d47689e00f8ecbc5d9f61bdb42e March 4, 2019 @ 20:23
MaxCDN 15434 bytes e1d98d47689e00f8ecbc5d9f61bdb42e (invalid)

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

Other Versions

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

4.3.1, 4.3.0,
4.2.1, 4.2.0,
4.1.3, 4.1.2, 4.1.1, 4.1.0,
4.0.0,
3.4.1, 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
14848 bytes -1 byte zopfli --i100000 --mls128 --bsr11 --lazy --ohh February 15, 2019 @ 15:05
14849 bytes -1 byte zopfli --i10000 --mls256 --bsr16 --lazy --ohh February 14, 2019 @ 23:51
14850 bytes -1 byte zopfli --i10000 --mls4096 --bsr8 --lazy --ohh February 14, 2019 @ 22:40
14851 bytes -1 byte zopfli --i1000 --mls2048 --bsr23 --lazy --ohh February 14, 2019 @ 20:18
14852 bytes -2 bytes zopfli --i1000 --mls1024 --bsr7 --lazy --ohh February 14, 2019 @ 20:09
14854 bytes zopfli --i100 --mls1024 --bsr4 --lazy --ohh February 14, 2019 @ 19:32

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

Most recent activity on March 4, 2019 @ 20:23.

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 or 100,000.
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
14858 14858 14858 14857 14851 14852 14852 14851 14852 14853 14857 14857 14859 14852 14869
14851 14857 14851 14852 14850 14851 14850 14851 14851 14851 14854 14852 14849 14852 14852
14852 14852 14851 14850 14850 14851 14848 14852 14854 14854 14852 14851 14853 14852 14852
14851 14859 14854 14850 14850 14851 14848 14850 14851 14850 14851 14851 14852 14852 14852
14851 14851 14851 14850 14851 14851 14848 14851 14853 14850 14851 14850 14851 14852 14852
14851 14861 14858 14850 14850 14852 14848 14851 14851 14849 14851 14851 14852 14852 14852
14850 14852 14852 14850 14850 14851 14850 14849 14852 14849 14850 14851 14852 14850 14851
14850 14860 14852 14851 14850 14851 14848 14849 14851 14850 14851 14851 14852 14851 14852
14851 14850 14851 14851 14851 14860 14851 14851 14853 14849 14851 14851 14852 14852 14852
14851 14851 14851 14851 14850 14851 14851 14851 14851 14850 14851 14851 14852 14852 14852
14851 14857 14851 14851 14850 14851 14850 14851 14851 14849 14850 14851 14852 14852 14852
14851 14859 14848 14850 14852 14851 14851 14850 14851 14850 14851 14852 14852 14852 14852
14851 14861 14851 14850 14851 14852 14851 14849 14850 14849 14851 14851 14853 14851 14852
14851 14858 14851 14850 14850 14861 14851 14851 14853 14851 14851 14851 14853 14852 14853
14850 14858 14851 14851 14851 14851 14848 14850 14850 14850 14850 14853 14852 14852 14852
14851 14851 14851 14850 14850 14851 14848 14850 14851 14849 14851 14851 14852 14852 14852
14851 14861 14851 14850 14851 14851 14851 14851 14851 14851 14851 14851 14852 14851 14852
14850 14861 14851 14851 14851 14850 14850 14848 14850 14850 14851 14851 14852 14852 14851
14850 14858 14851 14850 14850 14850 14851 14851 14851 14849 14851 14851 14852 14852 14852
14850 14858 14860 14851 14850 14851 14850 14851 14850 14850 14851 14851 14852 14852 14852
14850 14861 14852 14850 14850 14851 14848 14851 14851 14850 14851 14851 14852 14852 14852
14851 14860 14851 14850 14850 14851 14851 14850 14851 14849 14851 14851 14852 14852 14852
14851 14858 14851 14851 14850 14859 14851 14851 14850 14851 14851 14850 14852 14852 14853

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 14854 bytes 100%
1,000 14851 bytes -3 bytes 100%
10,000 14849 bytes -2 bytes 100%
100,000 14848 bytes -1 byte 6.38%
1,000,000
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
14917 bytes +69 bytes (+0.46%) +29 bytes
14913 bytes +65 bytes (+0.44%) +25 bytes
14888 bytes +40 bytes (+0.27%)
14889 bytes +41 bytes (+0.28%) +1 byte
14902 bytes +54 bytes (+0.36%) +14 bytes
14940 bytes +92 bytes (+0.62%) +52 bytes
14971 bytes +123 bytes (+0.83%) +83 bytes
15002 bytes +154 bytes (+1.04%) +114 bytes
15031 bytes +183 bytes (+1.23%) +143 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 12399 bytes -2449 bytes (-16.49%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12749 bytes -2099 bytes (-14.14%)
Brotli (Wikipedia) brotli brotli -q 11 13540 bytes -1308 bytes (-8.81%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 14337 bytes -511 bytes (-3.44%)
LZMA2 (Wikipedia) xz xz -9 14356 bytes -492 bytes (-3.31%)
Zstandard (Wikipedia) zstd zstd -19 14536 bytes -312 bytes (-2.10%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 14781 bytes -67 bytes (-0.45%)

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