Choose a version:
44% The original file has 143686 bytes (140.3k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 63240 bytes (61.8k, 44%).

After GZIP compression these minified files vary in size:
unpkg
  19143 bytes (18.7k)
CDN
gzip -6 (default)
  14942 bytes (14.6k)
local copy
jsdelivr
  14911 bytes (14.6k)
CDN
gzip -9
  14869 bytes (14.5k)
local copy
cdnjs
  14861 bytes (14.5k)
CDN
7zip -mx=9 -tgzip
  14380 bytes (14.0k)
local copy
libdeflate -12
  14366 bytes (14.0k)
local copy
zultra
  14351 bytes (14.0k)
local copy
kzip -s0 -rn -b2
  14341 bytes (14.0k)
local copy
pigz -11 -n
  14330 bytes (14.0k)
local copy
Zopfli
  14306 bytes (14.0k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.5.3.min.js (or via HTTPS)

You will automatically get the smallest Bootstrap 4.5.3 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 555 bytes by using my Bootstrap 4.5.3 Zopfli version instead of the best available CDN (3.88% smaller than cdnjs, 14306 vs. 14861 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 --mls8192 --bsr22 --lazy --ohh

(found October 19, 2020)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 8192  --mls8192
block splitting recursion 22  --bsr22
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/v4.5.3/dist/js/bootstrap.min.js --location | md5sum
f20fa8b102f205141295cdefd6ffe449  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.5.3.min.zopfli.js.gz | md5sum
f20fa8b102f205141295cdefd6ffe449  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/twbs/bootstrap/v4.5.3/dist/js/bootstrap.min.js --location | sha1sum
0c4e8445f6f0c9611dc1c13dc6f085eb4bcaca0b  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.5.3.min.zopfli.js.gz | sha1sum
0c4e8445f6f0c9611dc1c13dc6f085eb4bcaca0b  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
unpkg 19143 bytes f20fa8b102f205141295cdefd6ffe449 (invalid)
jsdelivr 14911 bytes f20fa8b102f205141295cdefd6ffe449 October 16, 2020 @ 13:50
cdnjs 14861 bytes f20fa8b102f205141295cdefd6ffe449 (invalid)

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

Other Versions

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

4.5.3, 4.5.2, 4.5.1, 4.5.0,
4.4.1, 4.4.0,
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
14306 bytes -1 byte zopfli --i1000000 --mls8192 --bsr22 --lazy --ohh October 19, 2020 @ 15:49
14307 bytes -1 byte zopfli --i100000 --mls16384 --bsr22 --lazy --ohh October 17, 2020 @ 09:43
14308 bytes -1 byte zopfli --i1000 --mls8192 --bsr19 --lazy --ohh October 16, 2020 @ 13:58
14309 bytes -2 bytes zopfli --i1000 --mls64 --bsr19 --lazy --ohh October 16, 2020 @ 13:57
14311 bytes zopfli --i100 --mls4096 --bsr14 --lazy --ohh October 16, 2020 @ 13:53

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

Most recent activity on October 20, 2020 @ 14:37.

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
14314 14315 14315 14315 14316 14316 14317 14316 14317 14318 14315 14323 14314 14326 14325
14312 14310 14310 14313 14312 14318 14312 14316 14321 14323 14310 14310 14306 14310 14312
14310 14311 14311 14316 14316 14308 14323 14313 14315 14323 14326 14311 14310 14309 14310
14309 14313 14312 14316 14317 14322 14312 14324 14315 14322 14325 14311 14310 14306 14310
14309 14317 14313 14313 14314 14313 14317 14323 14316 14316 14315 14311 14307 14306 14307
14309 14311 14313 14316 14317 14314 14316 14313 14316 14321 14324 14311 14310 14306 14309
14310 14311 14311 14310 14325 14313 14309 14313 14321 14323 14324 14310 14307 14309 14307
14309 14314 14312 14312 14312 14314 14313 14321 14322 14324 14316 14310 14310 14309 14309
14310 14311 14312 14316 14312 14324 14312 14315 14313 14324 14310 14311 14310 14309 14309
14310 14311 14312 14316 14314 14308 14324 14312 14315 14314 14324 14313 14310 14309 14309
14310 14310 14313 14310 14313 14312 14315 14324 14316 14316 14324 14307 14310 14310 14310
14310 14316 14312 14316 14314 14313 14312 14323 14315 14323 14323 14311 14310 14306 14310
14311 14314 14312 14309 14313 14313 14312 14313 14316 14323 14311 14311 14310 14309 14310
14311 14311 14313 14312 14313 14317 14315 14312 14316 14322 14311 14311 14310 14309 14310
14309 14311 14313 14317 14313 14313 14312 14323 14313 14316 14311 14311 14306 14309 14310
14309 14310 14312 14309 14314 14306 14324 14313 14315 14317 14327 14310 14306 14309 14309
14311 14313 14313 14316 14317 14313 14317 14323 14315 14314 14324 14310 14310 14306 14309
14311 14311 14311 14309 14312 14312 14312 14323 14315 14321 14325 14311 14311 14310 14307
14310 14310 14313 14313 14314 14312 14318 14324 14316 14324 14324 14311 14306 14306 14309
14310 14313 14312 14313 14314 14312 14312 14323 14316 14314 14325 14310 14310 14310 14307
14310 14310 14313 14316 14313 14312 14324 14325 14315 14315 14310 14311 14310 14306 14309
14311 14310 14313 14309 14314 14313 14312 14323 14317 14324 14310 14311 14310 14309 14309
14310 14311 14313 14309 14309 14312 14323 14313 14314 14324 14324 14310 14310 14309 14309

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 14311 bytes 100%
1,000 14308 bytes -3 bytes 100%
10,000 14308 bytes 100%
100,000 14307 bytes -1 byte 6.09%
1,000,000 14306 bytes -1 byte 5.51%
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
14378 bytes +72 bytes (+0.50%) +37 bytes
14378 bytes +72 bytes (+0.50%) +37 bytes
14341 bytes +35 bytes (+0.24%)
14356 bytes +50 bytes (+0.35%) +15 bytes
14349 bytes +43 bytes (+0.30%) +8 bytes
14366 bytes +60 bytes (+0.42%) +25 bytes
14378 bytes +72 bytes (+0.50%) +37 bytes
14421 bytes +115 bytes (+0.80%) +80 bytes
14457 bytes +151 bytes (+1.06%) +116 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 - for example, your browser actually supports it !
Algorithm Program Parameters Size Compared To Best Zopfli
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 11622 bytes -2684 bytes (-18.76%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12122 bytes -2184 bytes (-15.27%)
Brotli (Wikipedia) brotli brotli -q 11 13123 bytes -1183 bytes (-8.27%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 13632 bytes -674 bytes (-4.71%)
LZMA2 (Wikipedia) xz xz -9 13916 bytes -390 bytes (-2.73%)
Zstandard (Wikipedia) zstd zstd -19 14011 bytes -295 bytes (-2.06%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 14375 bytes +69 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 2020.
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

All trademarks are property of their respective owners. You know, the boring legal stuff.