Choose a version:
44% The original file has 127170 bytes (124.2k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 56292 bytes (55.0k, 44%).

After GZIP compression these minified files vary in size:
unpkg
  18435 bytes (18.0k)
CDN
cdnjs
  14823 bytes (14.5k)
CDN
gzip -6 (default)
  14674 bytes (14.3k)
local copy
MaxCDN
  14651 bytes (14.3k)
CDN
gzip -9
  14613 bytes (14.3k)
local copy
7zip -mx=9 -tgzip
  14139 bytes (13.8k)
local copy
libdeflate -12
  14128 bytes (13.8k)
local copy
kzip -s0 -rn -b2
  14120 bytes (13.8k)
local copy
pigz -11 -n
  14093 bytes (13.8k)
local copy
Zopfli
  14080 bytes (13.8k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.3.0.min.js (or via HTTPS)

You will automatically get the smallest Bootstrap 4.3.0 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 571 bytes by using my Bootstrap 4.3.0 Zopfli version instead of the best available CDN (4.06% smaller than MaxCDN, 14080 vs. 14651 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 --i10000 --mb8 --mls128 --bsr12 --lazy --ohh

(found February 14, 2019)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 128  --mls128
block splitting recursion 12  --bsr12
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/v4.3.0/dist/js/bootstrap.min.js --location | md5sum
988c3de826d51c758b2670b0e6a433d2  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.3.0.min.zopfli.js.gz | md5sum
988c3de826d51c758b2670b0e6a433d2  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/twbs/bootstrap/v4.3.0/dist/js/bootstrap.min.js --location | sha1sum
8bf989e0a7775b48d7f4454121204c2e05a9c6ce  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.3.0.min.zopfli.js.gz | sha1sum
8bf989e0a7775b48d7f4454121204c2e05a9c6ce  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
unpkg 18435 bytes 988c3de826d51c758b2670b0e6a433d2 (invalid)
cdnjs 14823 bytes 988c3de826d51c758b2670b0e6a433d2 (invalid)
MaxCDN 14651 bytes 988c3de826d51c758b2670b0e6a433d2 (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
14080 bytes -1 byte zopfli --i10000 --mls128 --bsr12 --lazy --ohh February 14, 2019 @ 22:52
14081 bytes -1 byte zopfli --i10000 --mls128 --bsr7 --lazy --ohh February 14, 2019 @ 16:38
14082 bytes -1 byte zopfli --i10000 --mls2 --bsr6 --lazy --ohh February 13, 2019 @ 20:23
14083 bytes -2 bytes zopfli --i1000 --mls2 --bsr16 --lazy --ohh February 13, 2019 @ 14:31
14085 bytes -1 byte zopfli --i1000 --mls2048 --bsr10 --lazy --ohh February 13, 2019 @ 14:30
14086 bytes zopfli --i100 --mls256 --bsr23 --lazy --ohh February 13, 2019 @ 13:26

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

Most recent activity on February 15, 2019 @ 18:49.

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
14083 14083 14085 14084 14084 14090 14083 14085 14084 14085 14084 14087 14084 14084 14108
14083 14083 14082 14084 14083 14095 14084 14083 14084 14084 14084 14090 14097 14084 14084
14082 14082 14082 14084 14083 14084 14085 14084 14084 14084 14084 14082 14084 14084 14095
14083 14083 14084 14083 14083 14083 14081 14084 14084 14085 14085 14082 14087 14082 14096
14082 14082 14083 14086 14083 14094 14083 14083 14082 14081 14083 14082 14084 14084 14083
14083 14083 14082 14082 14083 14094 14083 14084 14082 14082 14083 14087 14084 14084 14096
14093 14083 14084 14082 14083 14081 14083 14083 14086 14084 14084 14084 14084 14084 14096
14082 14082 14083 14082 14083 14083 14085 14084 14082 14086 14083 14083 14087 14083 14084
14083 14082 14082 14082 14083 14081 14080 14084 14082 14082 14083 14082 14085 14083 14083
14082 14082 14085 14083 14083 14093 14083 14083 14083 14083 14083 14082 14086 14084 14098
14083 14082 14088 14083 14083 14085 14081 14084 14082 14082 14086 14082 14084 14083 14083
14083 14083 14083 14082 14083 14083 14081 14082 14082 14082 14084 14082 14084 14083 14083
14083 14083 14083 14083 14084 14082 14083 14083 14091 14081 14085 14082 14085 14083 14083
14083 14083 14083 14082 14094 14083 14082 14083 14084 14085 14083 14082 14082 14085 14098
14083 14083 14083 14083 14084 14084 14083 14084 14086 14084 14083 14082 14087 14083 14084
14083 14084 14083 14086 14083 14084 14083 14084 14082 14082 14083 14083 14090 14083 14086
14083 14083 14082 14083 14083 14094 14084 14083 14083 14082 14083 14083 14083 14084 14083
14084 14082 14083 14083 14083 14082 14082 14083 14084 14082 14085 14083 14085 14084 14084
14082 14082 14083 14083 14083 14083 14083 14084 14083 14082 14085 14082 14084 14083 14097
14083 14083 14083 14083 14083 14095 14082 14083 14083 14084 14083 14083 14084 14083 14096
14083 14083 14083 14083 14083 14092 14083 14084 14083 14082 14084 14082 14084 14083 14096
14083 14083 14083 14082 14083 14083 14083 14084 14082 14082 14083 14082 14084 14084 14083
14082 14082 14083 14083 14083 14082 14084 14084 14090 14083 14087 14086 14084 14084 14097

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 14086 bytes 100%
1,000 14083 bytes -3 bytes 100%
10,000 14080 bytes -3 bytes 100%
100,000 14080 bytes 0.29%
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
14153 bytes +73 bytes (+0.52%) +33 bytes
14153 bytes +73 bytes (+0.52%) +33 bytes
14120 bytes +40 bytes (+0.28%)
14149 bytes +69 bytes (+0.49%) +29 bytes
14154 bytes +74 bytes (+0.53%) +34 bytes
14188 bytes +108 bytes (+0.77%) +68 bytes
14218 bytes +138 bytes (+0.98%) +98 bytes
14260 bytes +180 bytes (+1.28%) +140 bytes
14264 bytes +184 bytes (+1.31%) +144 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 11724 bytes -2356 bytes (-16.73%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12116 bytes -1964 bytes (-13.95%)
Brotli (Wikipedia) brotli brotli -q 11 12851 bytes -1229 bytes (-8.73%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 13611 bytes -469 bytes (-3.33%)
LZMA2 (Wikipedia) xz xz -9 13624 bytes -456 bytes (-3.24%)
Zstandard (Wikipedia) zstd zstd -19 13785 bytes -295 bytes (-2.10%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 14012 bytes -68 bytes (-0.48%)

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