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:
Microsoft
  20886 bytes (20.4k)
CDN
unpkg
  18691 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
  14233 bytes (13.9k)
local copy
zultra
  14229 bytes (13.9k)
local copy
pigz -11 -n
  14214 bytes (13.9k)
local copy
Zopfli
  14204 bytes (13.9k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.5.2.min.js (or via HTTPS)

You will automatically get the smallest Bootstrap 4.5.2 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 604 bytes by using my Bootstrap 4.5.2 Zopfli version instead of the best available CDN (4.25% smaller than jsdelivr, 14204 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 --i100000 --mb8 --mls4096 --bsr6 --lazy --ohh

(found August 11, 2020)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 4096  --mls4096
block splitting recursion 6  --bsr6
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/v4.5.2/dist/js/bootstrap.min.js --location | md5sum
02d223393e00c273efdcb1ade8f4f8b1  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.5.2.min.zopfli.js.gz | md5sum
02d223393e00c273efdcb1ade8f4f8b1  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/twbs/bootstrap/v4.5.2/dist/js/bootstrap.min.js --location | sha1sum
0cc93b8421d89c24a889642428b363cb831de78a  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.5.2.min.zopfli.js.gz | sha1sum
0cc93b8421d89c24a889642428b363cb831de78a  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Microsoft 20886 bytes 02d223393e00c273efdcb1ade8f4f8b1 August 13, 2020 @ 21:18
unpkg 18691 bytes 02d223393e00c273efdcb1ade8f4f8b1 (invalid)
cdnjs 14992 bytes 02d223393e00c273efdcb1ade8f4f8b1 (invalid)
MaxCDN 14811 bytes 02d223393e00c273efdcb1ade8f4f8b1 (invalid)
jsdelivr 14808 bytes 02d223393e00c273efdcb1ade8f4f8b1 August 10, 2020 @ 16:59

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

Other Versions

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

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
14204 bytes -1 byte zopfli --i100000 --mls4096 --bsr6 --lazy --ohh August 11, 2020 @ 14:13
14205 bytes -1 byte zopfli --i100000 --mls16384 --bsr8 --lazy --ohh August 11, 2020 @ 11:23
14206 bytes -1 byte zopfli --i10000 --mls16384 --bsr9 --lazy --ohh August 10, 2020 @ 17:40
14207 bytes -1 byte zopfli --i10000 --mls32 --bsr14 --lazy --ohh August 10, 2020 @ 17:31
14208 bytes -1 byte zopfli --i1000 --mls1024 --bsr8 --lazy --ohh August 10, 2020 @ 17:10
14209 bytes -2 bytes zopfli --i1000 --mls8192 --bsr17 --lazy --ohh August 10, 2020 @ 17:10
14211 bytes zopfli --i100 --mls8192 --bsr17 --lazy --ohh August 10, 2020 @ 17:02

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

Most recent activity on August 13, 2020 @ 21:18.

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
14211 14211 14211 14211 14210 14213 14213 14215 14213 14213 14215 14222 14222 14220 14224
14207 14209 14223 14208 14208 14209 14208 14207 14208 14208 14207 14207 14207 14208 14208
14209 14208 14208 14207 14208 14208 14210 14208 14208 14208 14207 14204 14207 14208 14228
14209 14211 14208 14208 14207 14207 14210 14208 14208 14207 14207 14207 14208 14209 14219
14208 14208 14208 14208 14208 14209 14208 14207 14208 14207 14207 14207 14207 14205 14209
14209 14209 14208 14208 14207 14209 14208 14208 14208 14207 14207 14208 14207 14206 14208
14209 14208 14208 14208 14208 14208 14208 14208 14209 14208 14207 14207 14208 14208 14207
14212 14208 14224 14209 14225 14205 14208 14208 14208 14209 14207 14208 14208 14206 14208
14208 14208 14208 14208 14209 14208 14208 14207 14208 14209 14207 14207 14207 14206 14205
14210 14209 14208 14225 14209 14209 14208 14209 14208 14207 14208 14207 14207 14208 14205
14208 14209 14208 14208 14207 14208 14207 14207 14208 14207 14207 14207 14207 14209 14205
14208 14209 14208 14207 14208 14208 14208 14208 14208 14207 14207 14207 14208 14209 14217
14210 14209 14208 14208 14208 14209 14208 14210 14208 14208 14208 14207 14205 14209 14205
14208 14209 14208 14208 14209 14208 14208 14208 14207 14208 14206 14207 14207 14206 14208
14209 14209 14208 14208 14207 14210 14208 14210 14208 14208 14208 14207 14207 14206 14207
14208 14208 14208 14208 14208 14208 14210 14207 14208 14207 14208 14207 14207 14207 14208
14208 14208 14208 14208 14208 14208 14207 14208 14208 14208 14207 14207 14205 14206 14205
14208 14210 14208 14208 14209 14209 14208 14208 14208 14207 14208 14208 14207 14206 14205
14209 14208 14208 14208 14208 14209 14208 14211 14208 14207 14207 14207 14205 14206 14205
14208 14208 14208 14208 14207 14209 14207 14210 14207 14207 14207 14207 14207 14206 14205
14208 14208 14208 14208 14208 14209 14208 14210 14208 14207 14208 14207 14207 14207 14205
14208 14208 14208 14208 14208 14209 14207 14208 14208 14207 14207 14207 14207 14206 14205
14209 14208 14208 14208 14208 14210 14208 14208 14208 14205 14208 14208 14207 14206 14205

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 14211 bytes 100%
1,000 14208 bytes -3 bytes 100%
10,000 14206 bytes -2 bytes 100%
100,000 14204 bytes -2 bytes 8.70%
1,000,000 14204 bytes 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 +73 bytes (+0.51%) +44 bytes
14279 bytes +75 bytes (+0.53%) +46 bytes
14233 bytes +29 bytes (+0.20%)
14245 bytes +41 bytes (+0.29%) +12 bytes
14236 bytes +32 bytes (+0.23%) +3 bytes
14268 bytes +64 bytes (+0.45%) +35 bytes
14315 bytes +111 bytes (+0.78%) +82 bytes
14347 bytes +143 bytes (+1.01%) +114 bytes
14360 bytes +156 bytes (+1.10%) +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 11625 bytes -2579 bytes (-18.16%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12097 bytes -2107 bytes (-14.83%)
Brotli (Wikipedia) brotli brotli -q 11 13016 bytes -1188 bytes (-8.36%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 13561 bytes -643 bytes (-4.53%)
LZMA2 (Wikipedia) xz xz -9 13800 bytes -404 bytes (-2.84%)
Zstandard (Wikipedia) zstd zstd -19 13931 bytes -273 bytes (-1.92%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 14293 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.