Choose a version:
44% The original file has 136323 bytes (133.1k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 60174 bytes (58.8k, 44%).

After GZIP compression these minified files vary in size:
Microsoft
  20954 bytes (20.5k)
CDN
unpkg
  18751 bytes (18.3k)
CDN
cdnjs
  15061 bytes (14.7k)
CDN
gzip -6 (default)
  14908 bytes (14.6k)
local copy
MaxCDN
  14885 bytes (14.5k)
CDN
jsdelivr
  14885 bytes (14.5k)
CDN
gzip -9
  14862 bytes (14.5k)
local copy
7zip -mx=9 -tgzip
  14338 bytes (14.0k)
local copy
libdeflate -12
  14331 bytes (14.0k)
local copy
kzip -s0 -rn -b2
  14304 bytes (14.0k)
local copy
pigz -11 -n
  14287 bytes (14.0k)
local copy
Zopfli
  14275 bytes (13.9k)
local copy
Zopfli (defluff)
  14274 bytes (13.9k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.5.0.min.js (or via HTTPS)

You will automatically get the smallest Bootstrap 4.5.0 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 610 bytes by using my Bootstrap 4.5.0 Zopfli version instead of the best available CDN (4.27% smaller than jsdelivr, 14275 vs. 14885 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 --bsr6 --lazy --ohh

(found May 15, 2020)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 8192  --mls8192
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

Even Smaller Files Thanks To Defluff

Zopfli's output can be further optimized by the defluff tool.
In this particular case, defluff saves 1 more byte (14274 bytes).

Verify file integrity

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

MD5:
curl --silent --compressed https://raw.githubusercontent.com/twbs/bootstrap/v4.5.0/dist/js/bootstrap.min.js --location | md5sum
6bea60c34c5db6797150610dacdc6bce  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.5.0.min.zopfli.js.gz | md5sum
6bea60c34c5db6797150610dacdc6bce  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/twbs/bootstrap/v4.5.0/dist/js/bootstrap.min.js --location | sha1sum
544afefd148715da7dd52d368a414703390ca0e0  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-4.5.0.min.zopfli.js.gz | sha1sum
544afefd148715da7dd52d368a414703390ca0e0  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Microsoft 20954 bytes 6bea60c34c5db6797150610dacdc6bce May 19, 2020 @ 17:58
unpkg 18751 bytes 6bea60c34c5db6797150610dacdc6bce (invalid)
cdnjs 15061 bytes 6bea60c34c5db6797150610dacdc6bce (invalid)
MaxCDN 14885 bytes 6bea60c34c5db6797150610dacdc6bce (invalid)
jsdelivr 14885 bytes 6bea60c34c5db6797150610dacdc6bce May 14, 2020 @ 14:21

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

Other Versions

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

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
14275 bytes -1 byte zopfli --i1000000 --mls8192 --bsr6 --lazy --ohh May 15, 2020 @ 13:49
14276 bytes -1 byte zopfli --i100000 --mls4096 --bsr17 --lazy --ohh May 14, 2020 @ 18:20
14277 bytes -1 byte zopfli --i10000 --mls4096 --bsr19 --lazy --ohh May 14, 2020 @ 15:17
14278 bytes -2 bytes zopfli --i10000 --mls16384 --bsr15 --lazy --ohh May 14, 2020 @ 15:17
14280 bytes -2 bytes zopfli --i1000 --mls4096 --bsr10 --lazy --ohh May 14, 2020 @ 15:03
14282 bytes zopfli --i100 --mls4096 --bsr17 --lazy --ohh May 14, 2020 @ 14:26

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

Most recent activity on May 19, 2020 @ 17:58.

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
14285 14284 14284 14283 14284 14283 14284 14284 14282 14285 14286 14285 14300 14296 14296
14285 14280 14281 14285 14285 14280 14283 14282 14283 14281 14280 14281 14280 14281 14282
14282 14281 14281 14285 14283 14281 14280 14283 14281 14281 14277 14280 14275 14281 14280
14282 14281 14281 14283 14282 14281 14280 14282 14281 14281 14281 14282 14280 14277 14280
14282 14281 14282 14282 14281 14279 14283 14282 14282 14281 14282 14281 14280 14277 14280
14281 14280 14283 14283 14282 14281 14283 14283 14281 14281 14280 14282 14280 14280 14281
14283 14279 14281 14281 14282 14282 14283 14283 14281 14280 14281 14276 14280 14277 14281
14281 14282 14283 14282 14282 14279 14283 14283 14281 14283 14280 14281 14277 14281 14281
14282 14281 14283 14282 14281 14280 14284 14284 14281 14281 14281 14281 14280 14277 14281
14281 14280 14282 14282 14281 14283 14283 14282 14283 14281 14281 14281 14280 14280 14280
14282 14281 14282 14282 14282 14279 14283 14282 14281 14281 14281 14281 14281 14284 14282
14281 14279 14283 14282 14281 14280 14283 14282 14281 14281 14281 14282 14280 14277 14282
14281 14280 14281 14281 14281 14280 14283 14283 14282 14281 14281 14282 14282 14282 14291
14282 14281 14283 14283 14281 14281 14283 14281 14281 14281 14281 14275 14296 14277 14282
14282 14280 14281 14282 14282 14282 14283 14283 14281 14281 14281 14282 14281 14284 14281
14282 14281 14282 14280 14281 14282 14283 14283 14282 14281 14281 14276 14281 14280 14281
14281 14281 14283 14282 14282 14280 14283 14282 14281 14281 14280 14281 14280 14284 14281
14281 14281 14283 14282 14281 14283 14280 14282 14282 14281 14281 14281 14294 14281 14281
14282 14281 14283 14282 14282 14283 14283 14282 14281 14281 14281 14276 14280 14277 14281
14282 14280 14281 14282 14282 14283 14280 14282 14281 14281 14281 14282 14280 14283 14281
14281 14280 14282 14282 14281 14283 14283 14283 14281 14281 14281 14282 14294 14280 14281
14282 14281 14281 14282 14282 14279 14283 14282 14281 14281 14281 14282 14280 14284 14281
14282 14281 14282 14282 14282 14279 14283 14284 14283 14280 14280 14281 14277 14284 14281

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 14282 bytes 100%
1,000 14280 bytes -2 bytes 100%
10,000 14277 bytes -3 bytes 100%
100,000 14276 bytes -1 byte 4.35%
1,000,000 14275 bytes -1 byte 1.45%
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
14350 bytes +75 bytes (+0.53%) +46 bytes
14349 bytes +74 bytes (+0.52%) +45 bytes
14304 bytes +29 bytes (+0.20%)
14327 bytes +52 bytes (+0.36%) +23 bytes
14321 bytes +46 bytes (+0.32%) +17 bytes
14343 bytes +68 bytes (+0.48%) +39 bytes
14351 bytes +76 bytes (+0.53%) +47 bytes
14398 bytes +123 bytes (+0.86%) +94 bytes
14429 bytes +154 bytes (+1.08%) +125 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 11692 bytes -2583 bytes (-18.09%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 12130 bytes -2145 bytes (-15.03%)
Brotli (Wikipedia) brotli brotli -q 11 13079 bytes -1196 bytes (-8.38%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 13642 bytes -633 bytes (-4.43%)
LZMA2 (Wikipedia) xz xz -9 13892 bytes -383 bytes (-2.68%)
Zstandard (Wikipedia) zstd zstd -19 13992 bytes -283 bytes (-1.98%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 14356 bytes +81 bytes (+0.57%)

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.