Choose a version:
40% The original file has 148892 bytes (145.4k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 59219 bytes (57.8k, 40%).

After GZIP compression these minified files vary in size:
unpkg
  22684 bytes (22.2k)
CDN
gzip -6 (default)
  16368 bytes (16.0k)
local copy
jsdelivr
  16337 bytes (16.0k)
CDN
cdnjs
  16291 bytes (15.9k)
CDN
gzip -9
  16287 bytes (15.9k)
local copy
7zip -mx=9 -tgzip
  15770 bytes (15.4k)
local copy
libdeflate -12
  15735 bytes (15.4k)
local copy
zultra
  15730 bytes (15.4k)
local copy
kzip -s0 -rn -b2
  15714 bytes (15.3k)
local copy
pigz -11 -n
  15706 bytes (15.3k)
local copy
Zopfli
  15694 bytes (15.3k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/bootstrap/bootstrap-5.1.2.min.js (or via HTTPS)

You will automatically get the smallest Bootstrap 5.1.2 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 597 bytes by using my Bootstrap 5.1.2 Zopfli version instead of the best available CDN (3.80% smaller than cdnjs, 15694 vs. 16291 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 --mls8 --bsr20 --lazy --ohh

(found October 6, 2021)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 8  --mls8
block splitting recursion 20  --bsr20
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/v5.1.2/dist/js/bootstrap.min.js --location | md5sum
6fb107c3f0d2309a7a29cb5c25cf6984  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-5.1.2.min.zopfli.js.gz | md5sum
6fb107c3f0d2309a7a29cb5c25cf6984  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/twbs/bootstrap/v5.1.2/dist/js/bootstrap.min.js --location | sha1sum
59c01b1dafbff713bc203f082da0963fdda204f4  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-5.1.2.min.zopfli.js.gz | sha1sum
59c01b1dafbff713bc203f082da0963fdda204f4  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
unpkg 22684 bytes 6fb107c3f0d2309a7a29cb5c25cf6984 (invalid)
jsdelivr 16337 bytes 6fb107c3f0d2309a7a29cb5c25cf6984 October 6, 2021 @ 11:42
cdnjs 16291 bytes 6fb107c3f0d2309a7a29cb5c25cf6984 (invalid)

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

Other Versions

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

5.1.3, 5.1.2, 5.1.1, 5.1.0,
5.0.2, 5.0.1, 5.0.0,
4.6.0,
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
15694 bytes -1 byte zopfli --i100000 --mls8 --bsr20 --lazy --ohh October 6, 2021 @ 12:58
15695 bytes -1 byte zopfli --i1000 --mls8 --bsr15 --lazy --ohh October 6, 2021 @ 11:53
15696 bytes -1 byte zopfli --i1000 --mls16 --bsr17 --lazy --ohh October 6, 2021 @ 11:51
15697 bytes zopfli --i100 --mls32 --bsr21 --lazy --ohh October 6, 2021 @ 11:46

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

Most recent activity on October 8, 2021 @ 02:32.

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
15713 15713 15706 15706 15712 15705 15707 15708 15713 15707 15705 15704 15703 15713 15713
15703 15703 15703 15695 15695 15698 15697 15697 15698 15695 15697 15698 15699 15699 15700
15700 15700 15696 15697 15694 15698 15696 15698 15705 15696 15697 15696 15699 15700 15701
15696 15698 15694 15697 15698 15698 15696 15697 15698 15696 15696 15705 15700 15699 15699
15698 15699 15696 15697 15695 15699 15697 15697 15697 15697 15696 15696 15698 15699 15699
15698 15698 15696 15694 15695 15699 15696 15697 15697 15696 15696 15697 15703 15699 15697
15700 15699 15697 15697 15695 15699 15694 15698 15697 15699 15697 15699 15700 15700 15699
15698 15700 15699 15697 15694 15698 15697 15704 15698 15697 15696 15697 15698 15700 15699
15699 15699 15697 15698 15694 15699 15699 15697 15697 15697 15696 15696 15696 15698 15700
15698 15704 15697 15697 15699 15695 15698 15697 15697 15697 15696 15696 15698 15699 15707
15698 15698 15697 15697 15695 15699 15696 15697 15697 15696 15696 15696 15698 15700 15702
15698 15701 15694 15697 15695 15697 15696 15697 15697 15696 15696 15696 15699 15700 15699
15705 15704 15704 15694 15704 15705 15704 15704 15697 15696 15696 15697 15699 15700 15700
15700 15699 15694 15696 15695 15698 15696 15696 15697 15696 15696 15705 15698 15700 15699
15698 15702 15697 15695 15697 15699 15696 15697 15697 15696 15697 15697 15698 15700 15700
15698 15698 15696 15696 15699 15704 15696 15696 15696 15696 15697 15697 15698 15700 15699
15698 15700 15694 15696 15694 15700 15698 15697 15697 15697 15697 15698 15698 15699 15698
15698 15698 15698 15697 15694 15700 15697 15697 15698 15696 15697 15696 15699 15700 15700
15699 15699 15694 15694 15694 15699 15696 15697 15697 15697 15694 15698 15698 15700 15697
15700 15699 15698 15699 15694 15699 15697 15696 15697 15696 15697 15703 15701 15699 15699
15698 15699 15699 15697 15695 15699 15698 15697 15697 15696 15697 15697 15699 15701 15700
15698 15699 15696 15695 15695 15699 15696 15705 15697 15696 15696 15696 15698 15699 15699
15698 15701 15695 15695 15696 15698 15697 15696 15697 15697 15697 15697 15699 15699 15698

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 15697 bytes 100%
1,000 15695 bytes -2 bytes 100%
10,000 15695 bytes 100%
100,000 15694 bytes -1 byte 9.57%
1,000,000 15694 bytes 4.93%
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
15788 bytes +94 bytes (+0.60%) +74 bytes
15788 bytes +94 bytes (+0.60%) +74 bytes
15714 bytes +20 bytes (+0.13%)
15739 bytes +45 bytes (+0.29%) +25 bytes
15766 bytes +72 bytes (+0.46%) +52 bytes
15788 bytes +94 bytes (+0.60%) +74 bytes
15807 bytes +113 bytes (+0.72%) +93 bytes
15846 bytes +152 bytes (+0.97%) +132 bytes
15881 bytes +187 bytes (+1.19%) +167 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 13289 bytes -2405 bytes (-15.32%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 13619 bytes -2075 bytes (-13.22%)
Brotli (Wikipedia) brotli brotli -q 11 14591 bytes -1103 bytes (-7.03%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 15356 bytes -338 bytes (-2.15%)
LZMA2 (Wikipedia) xz xz -9 15456 bytes -238 bytes (-1.52%)
Zstandard (Wikipedia) zstd zstd -19 15568 bytes -126 bytes (-0.80%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 15759 bytes +65 bytes (+0.41%)

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 2021.
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.