Choose a version:
44% The original file has 135565 bytes (132.4k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 60044 bytes (58.6k, 44%).

After GZIP compression these minified files vary in size:
unpkg
  18690 bytes (18.3k)
CDN
cdnjs
  14992 bytes (14.6k)
CDN
gzip -6 (default)
  14834 bytes (14.5k)
local copy
MaxCDN
  14811 bytes (14.5k)
CDN
jsdelivr
  14808 bytes (14.5k)
CDN
gzip -9
  14778 bytes (14.4k)
local copy
7zip -mx=9 -tgzip
  14277 bytes (13.9k)
local copy
libdeflate -12
  14261 bytes (13.9k)
local copy
kzip -s0 -rn -b2
  14234 bytes (13.9k)
local copy
zultra
  14228 bytes (13.9k)
local copy
pigz -11 -n
  14214 bytes (13.9k)
local copy
Zopfli
  14203 bytes (13.9k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.5.1.min.js (or via HTTPS)

You will automatically get the smallest Bootstrap 4.5.1 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 605 bytes by using my Bootstrap 4.5.1 Zopfli version instead of the best available CDN (4.26% smaller than jsdelivr, 14203 vs. 14808 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 --mls4096 --bsr21 --lazy --ohh

(found August 11, 2020)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 4096  --mls4096
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

Verify file integrity

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

MD5:
curl --silent --compressed https://raw.githubusercontent.com/twbs/bootstrap/v4.5.1/dist/js/bootstrap.min.js --location | md5sum
aa073ec0db74966cc0dcf16632b2bd07  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.5.1.min.zopfli.js.gz | md5sum
aa073ec0db74966cc0dcf16632b2bd07  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/twbs/bootstrap/v4.5.1/dist/js/bootstrap.min.js --location | sha1sum
a52652263d8f72dd9125a982d76f6ce96b454ca2  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.5.1.min.zopfli.js.gz | sha1sum
a52652263d8f72dd9125a982d76f6ce96b454ca2  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
unpkg 18690 bytes aa073ec0db74966cc0dcf16632b2bd07 (invalid)
cdnjs 14992 bytes aa073ec0db74966cc0dcf16632b2bd07 (invalid)
MaxCDN 14811 bytes aa073ec0db74966cc0dcf16632b2bd07 (invalid)
jsdelivr 14808 bytes aa073ec0db74966cc0dcf16632b2bd07 August 10, 2020 @ 16:58

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
14203 bytes -1 byte zopfli --i1000000 --mls4096 --bsr21 --lazy --ohh August 11, 2020 @ 18:51
14204 bytes -1 byte zopfli --i100000 --mls4096 --bsr21 --lazy --ohh August 11, 2020 @ 11:23
14205 bytes -1 byte zopfli --i10000 --mls32768 --bsr23 --lazy --ohh August 10, 2020 @ 18:38
14206 bytes -1 byte zopfli --i1000 --mls2048 --bsr14 --lazy --ohh August 10, 2020 @ 17:37
14207 bytes -2 bytes zopfli --i1000 --mls512 --bsr12 --lazy --ohh August 10, 2020 @ 17:11
14209 bytes -1 byte zopfli --i1000 --mls64 --bsr10 --lazy --ohh August 10, 2020 @ 17:10
14210 bytes zopfli --i100 --mls8192 --bsr40 --lazy --ohh August 10, 2020 @ 17:01

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

Most recent activity on August 11, 2020 @ 18:51.

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
14210 14211 14210 14211 14210 14213 14213 14215 14213 14212 14215 14222 14221 14220 14225
14221 14209 14222 14207 14207 14209 14207 14206 14207 14207 14206 14207 14207 14207 14206
14209 14207 14207 14207 14207 14208 14210 14208 14206 14206 14206 14206 14225 14207 14228
14209 14208 14208 14207 14207 14206 14210 14206 14207 14206 14206 14207 14205 14206 14204
14207 14207 14207 14207 14207 14209 14207 14206 14207 14206 14206 14204 14204 14204 14207
14209 14208 14207 14207 14206 14208 14208 14206 14207 14206 14207 14207 14206 14204 14221
14206 14208 14207 14207 14207 14207 14207 14207 14209 14207 14206 14206 14207 14208 14207
14207 14207 14223 14208 14225 14205 14207 14207 14207 14209 14207 14207 14207 14204 14207
14207 14206 14207 14208 14209 14207 14207 14206 14206 14209 14206 14204 14205 14204 14204
14209 14209 14207 14207 14208 14208 14208 14209 14207 14206 14207 14206 14226 14207 14204
14208 14209 14207 14207 14206 14207 14206 14206 14207 14206 14206 14207 14206 14206 14204
14208 14211 14208 14205 14207 14207 14208 14206 14207 14205 14206 14206 14206 14207 14204
14209 14209 14207 14207 14207 14209 14207 14209 14207 14206 14207 14207 14206 14207 14207
14207 14208 14207 14207 14208 14206 14207 14206 14206 14207 14205 14206 14204 14207 14207
14209 14208 14208 14207 14206 14210 14207 14210 14207 14207 14205 14207 14207 14204 14204
14207 14207 14207 14207 14207 14205 14209 14206 14206 14206 14207 14207 14226 14208 14207
14208 14207 14207 14208 14207 14207 14206 14206 14207 14206 14206 14206 14206 14207 14207
14207 14209 14207 14208 14209 14209 14208 14205 14207 14206 14207 14203 14225 14207 14207
14208 14207 14208 14207 14208 14209 14207 14210 14207 14206 14206 14204 14204 14204 14204
14208 14207 14207 14207 14205 14209 14206 14209 14206 14206 14206 14204 14204 14204 14204
14207 14207 14207 14207 14208 14208 14207 14210 14207 14206 14209 14204 14206 14204 14204
14208 14207 14209 14207 14208 14208 14206 14206 14207 14206 14206 14206 14226 14204 14204
14209 14207 14207 14207 14208 14210 14207 14208 14208 14206 14206 14207 14207 14207 14204

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 14210 bytes 100%
1,000 14206 bytes -4 bytes 100%
10,000 14205 bytes -1 byte 100%
100,000 14204 bytes -1 byte 11.88%
1,000,000 14203 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
14277 bytes +74 bytes (+0.52%) +43 bytes
14276 bytes +73 bytes (+0.51%) +42 bytes
14234 bytes +31 bytes (+0.22%)
14247 bytes +44 bytes (+0.31%) +13 bytes
14238 bytes +35 bytes (+0.25%) +4 bytes
14267 bytes +64 bytes (+0.45%) +33 bytes
14313 bytes +110 bytes (+0.77%) +79 bytes
14348 bytes +145 bytes (+1.02%) +114 bytes
14361 bytes +158 bytes (+1.11%) +127 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 11621 bytes -2582 bytes (-18.18%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12097 bytes -2106 bytes (-14.83%)
Brotli (Wikipedia) brotli brotli -q 11 13015 bytes -1188 bytes (-8.36%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 13581 bytes -622 bytes (-4.38%)
LZMA2 (Wikipedia) xz xz -9 13808 bytes -395 bytes (-2.78%)
Zstandard (Wikipedia) zstd zstd -19 13931 bytes -272 bytes (-1.92%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 14292 bytes +89 bytes (+0.63%)

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.