Choose a version:
53% The original file has 75484 bytes (73.7k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 39680 bytes (38.8k, 53%).

After GZIP compression these minified files vary in size:
unpkg
  13530 bytes (13.2k)
CDN
gzip -6 (default)
  10963 bytes (10.7k)
local copy
MaxCDN
  10940 bytes (10.7k)
CDN
gzip -9
  10919 bytes (10.7k)
local copy
libdeflate -12
  10574 bytes (10.3k)
local copy
7zip -mx=9 -tgzip
  10573 bytes (10.3k)
local copy
kzip -s0 -rn -b0
  10557 bytes (10.3k)
local copy
pigz -11 -n
  10525 bytes (10.3k)
local copy
Zopfli
  10513 bytes (10.3k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/bootstrap/bootstrap-3.4.1.min.js (or via HTTPS)

You will automatically get the smallest Bootstrap 3.4.1 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 427 bytes by using my Bootstrap 3.4.1 Zopfli version instead of the best available CDN (4.06% smaller than MaxCDN, 10513 vs. 10940 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 --i10000 --mb8 --mls2 --bsr6 --lazy --ohh

(found February 14, 2019)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 2  --mls2
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/v3.4.1/dist/js/bootstrap.min.js --location | md5sum
2f34b630ffe30ba2ff2b91e3f3c322a1  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-3.4.1.min.zopfli.js.gz | md5sum
2f34b630ffe30ba2ff2b91e3f3c322a1  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/twbs/bootstrap/v3.4.1/dist/js/bootstrap.min.js --location | sha1sum
b16fd8226bd6bfb08e568f1b1d0a21d60247cefb  -
curl --silent --compressed https://minime.stephan-brumme.com/files/bootstrap/bootstrap-3.4.1.min.zopfli.js.gz | sha1sum
b16fd8226bd6bfb08e568f1b1d0a21d60247cefb  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
unpkg 13530 bytes 2f34b630ffe30ba2ff2b91e3f3c322a1 (invalid)
MaxCDN 10940 bytes 2f34b630ffe30ba2ff2b91e3f3c322a1 (invalid)

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

Other Versions

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

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
10513 bytes -1 byte zopfli --i10000 --mls2 --bsr6 --lazy --ohh February 14, 2019 @ 23:26
10514 bytes -1 byte zopfli --i10000 --mls256 --bsr6 --lazy --ohh February 14, 2019 @ 23:14
10515 bytes zopfli --i100 --mls256 --bsr17 --lazy --ohh February 14, 2019 @ 19:27

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

Most recent activity on February 16, 2019 @ 05:20.

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 or 100,000.
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
10521 10521 10521 10521 10521 10523 10520 10520 10521 10520 10519 10521 10521 10525 10521
10517 10515 10518 10516 10516 10515 10516 10516 10515 10515 10515 10515 10516 10516 10519
10513 10514 10514 10514 10517 10515 10516 10514 10515 10514 10521 10516 10515 10517 10514
10515 10514 10514 10514 10518 10513 10519 10518 10515 10517 10516 10515 10515 10515 10517
10517 10515 10515 10514 10518 10513 10515 10515 10516 10516 10517 10514 10515 10517 10520
10520 10514 10515 10514 10518 10513 10516 10516 10515 10516 10516 10514 10516 10516 10514
10516 10515 10514 10515 10518 10516 10516 10516 10516 10515 10517 10514 10516 10517 10514
10516 10514 10515 10515 10518 10516 10519 10518 10516 10517 10518 10518 10516 10516 10516
10517 10515 10515 10515 10518 10516 10516 10516 10516 10516 10515 10514 10515 10517 10514
10514 10514 10515 10514 10518 10513 10516 10516 10516 10516 10517 10516 10515 10517 10514
10516 10515 10516 10514 10518 10514 10516 10516 10517 10516 10517 10516 10514 10516 10515
10515 10514 10515 10514 10517 10515 10517 10514 10514 10517 10517 10517 10515 10516 10516
10516 10514 10520 10514 10518 10515 10516 10515 10514 10516 10518 10516 10514 10517 10517
10516 10515 10516 10515 10518 10513 10516 10514 10515 10516 10516 10518 10514 10517 10518
10520 10516 10516 10515 10518 10513 10516 10515 10515 10516 10518 10514 10516 10515 10516
10516 10514 10515 10515 10517 10513 10516 10514 10516 10515 10517 10514 10515 10515 10516
10516 10514 10514 10515 10517 10514 10515 10516 10514 10514 10517 10516 10514 10517 10514
10514 10514 10515 10515 10518 10513 10516 10514 10516 10517 10517 10514 10515 10517 10516
10517 10514 10515 10515 10518 10515 10514 10516 10515 10516 10517 10514 10514 10517 10516
10517 10515 10515 10515 10518 10513 10516 10516 10515 10514 10517 10516 10516 10516 10516
10515 10515 10515 10516 10518 10513 10516 10516 10516 10516 10517 10514 10515 10517 10515
10514 10514 10516 10515 10518 10514 10515 10515 10515 10516 10517 10514 10515 10517 10515
10515 10515 10515 10514 10517 10516 10514 10516 10514 10516 10518 10514 10514 10517 10516

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 10515 bytes 100%
1,000 10515 bytes 100%
10,000 10513 bytes -2 bytes 100%
100,000 10513 bytes 3.19%
1,000,000
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
10557 bytes +44 bytes (+0.42%)
10557 bytes +44 bytes (+0.42%)
10567 bytes +54 bytes (+0.51%) +10 bytes
10561 bytes +48 bytes (+0.46%) +4 bytes
10589 bytes +76 bytes (+0.72%) +32 bytes
10600 bytes +87 bytes (+0.83%) +43 bytes
10642 bytes +129 bytes (+1.23%) +85 bytes
10685 bytes +172 bytes (+1.64%) +128 bytes
10714 bytes +201 bytes (+1.91%) +157 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 - but your browser doesn't support it.
Algorithm Program Parameters Size Compared To Best Zopfli
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 8880 bytes -1633 bytes (-15.53%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 9480 bytes -1033 bytes (-9.83%)
Brotli (Wikipedia) brotli brotli -q 11 9682 bytes -831 bytes (-7.90%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 9979 bytes -534 bytes (-5.08%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 10313 bytes -200 bytes (-1.90%)
LZMA2 (Wikipedia) xz xz -9 10396 bytes -117 bytes (-1.11%)
Zstandard (Wikipedia) zstd zstd -19 10423 bytes -90 bytes (-0.86%)

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 2019.
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