Choose a version:
31% The original file has 58319 bytes (57.0k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 18069 bytes (17.6k, 31%).

After GZIP compression these minified files vary in size:
Boot
  6617 bytes (6.5k)
CDN
cdnjs
  6465 bytes (6.3k)
CDN
gzip -6 (default)
  6460 bytes (6.3k)
local copy
gzip -9
  6452 bytes (6.3k)
local copy
unpkg
  6428 bytes (6.3k)
CDN
7zip -mx=9 -tgzip
  6260 bytes (6.1k)
local copy
libdeflate -12
  6256 bytes (6.1k)
local copy
kzip -s0 -rn -b0
  6252 bytes (6.1k)
local copy
pigz -11 -n
  6247 bytes (6.1k)
local copy
Zopfli
  6239 bytes (6.1k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.9.1.min.js

You will automatically get the smallest UnderscoreJS 1.9.1 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 189 bytes by using my UnderscoreJS 1.9.1 Zopfli version instead of the best available CDN (3.03% smaller than unpkg, 6239 vs. 6428 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 --mls256 --bsr8 --lazy --ohh

(found June 4, 2018)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 256  --mls256
block splitting recursion 8  --bsr8
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/jashkenas/underscore/1.9.1/underscore-min.js --location | md5sum
66e46fdc840c68d4ba9d8c6bd6ea9cfa  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.9.1.min.zopfli.js.gz | md5sum
66e46fdc840c68d4ba9d8c6bd6ea9cfa  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.9.1/underscore-min.js --location | sha1sum
69b8b33acbd62281920d4b73c310c66c4cfc8992  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.9.1.min.zopfli.js.gz | sha1sum
69b8b33acbd62281920d4b73c310c66c4cfc8992  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 6617 bytes 66e46fdc840c68d4ba9d8c6bd6ea9cfa June 7, 2018 @ 09:35
cdnjs 6465 bytes 66e46fdc840c68d4ba9d8c6bd6ea9cfa (invalid)
unpkg 6428 bytes 66e46fdc840c68d4ba9d8c6bd6ea9cfa (invalid)

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

Other Versions

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

1.9.1, 1.9.0,
1.8.3, 1.8.2, 1.8.1,
1.7.0,
1.6.0,
1.5.2, 1.5.0,
1.4.4, 1.4.3, 1.4.2, 1.4.1, 1.4.0,
1.3.3, 1.3.2, 1.3.1, 1.3.0,
1.2.4, 1.2.3, 1.2.2, 1.2.1, 1.2.0,
1.1.7, 1.1.5, 1.1.4, 1.1.3, 1.1.2, 1.1.1, 1.1.0,
1.0.4, 1.0.3, 1.0.2, 1.0.1, 1.0.0,
0.6.0,
0.5.7, 0.5.6, 0.5.5, 0.5.4, 0.5.3, 0.5.2, 0.5.1, 0.5.0,
0.4.7, 0.4.6, 0.4.5, 0.4.4, 0.4.3, 0.4.2, 0.4.1, 0.4.0,
0.3.3, 0.3.2, 0.3.1, 0.3.0,
0.2.0,
0.1.1, 0.1.0

The project site contains an overview how well these versions were compressed.
Other interesting projects are AngularJS, BackboneJS, Bootstrap, D3, Dojo, Ember, jQuery, Knockout, lodash, React, Socket.IO, ThreeJS and Vue.

Changelog

Best Zopfli parameters so far:
Size Improvement Parameters Found
6239 bytes -1 byte zopfli --i100000 --mls256 --bsr8 --lazy --ohh June 4, 2018 @ 12:01
6240 bytes -2 bytes zopfli --i1000 --mls256 --bsr14 --lazy --ohh June 4, 2018 @ 09:34
6242 bytes zopfli --i100 --mls1024 --bsr14 --lazy --ohh June 4, 2018 @ 09:12

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

Most recent activity on June 7, 2018 @ 09:35.

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
6251 6251 6260 6251 6251 6251 6251 6249 6251 6251 6251 6242 6251 6250 6250
6254 6254 6258 6256 6258 6253 6245 6257 6245 6244 6244 6249 6243 6244 6243
6258 6258 6256 6258 6256 6253 6257 6244 6244 6246 6243 6244 6245 6248 6248
6258 6258 6258 6258 6256 6245 6256 6248 6244 6240 6246 6247 6247 6244 6253
6253 6253 6255 6253 6251 6250 6247 6239 6243 6244 6244 6242 6244 6247 6248
6256 6256 6258 6245 6256 6241 6243 6251 6240 6243 6244 6245 6246 6244 6244
6258 6258 6254 6256 6256 6257 6257 6252 6240 6244 6244 6240 6244 6244 6244
6244 6244 6255 6258 6244 6244 6243 6245 6244 6244 6244 6243 6244 6244 6244
6257 6257 6256 6258 6256 6244 6256 6257 6245 6244 6244 6247 6246 6244 6244
6257 6257 6256 6256 6256 6255 6256 6243 6243 6244 6243 6244 6245 6246 6250
6253 6253 6256 6242 6251 6253 6253 6240 6242 6240 6244 6247 6246 6246 6246
6258 6258 6258 6256 6255 6244 6257 6248 6243 6244 6244 6247 6244 6246 6246
6256 6256 6255 6257 6256 6256 6255 6257 6243 6244 6244 6246 6247 6247 6247
6258 6258 6256 6255 6256 6250 6256 6248 6240 6243 6244 6244 6244 6247 6247
6258 6258 6255 6260 6257 6254 6244 6248 6244 6244 6244 6244 6247 6244 6244
6254 6254 6256 6243 6255 6244 6244 6244 6245 6241 6244 6244 6247 6250 6244
6256 6256 6255 6258 6257 6255 6255 6256 6243 6243 6244 6244 6244 6244 6244
6257 6257 6256 6256 6256 6258 6257 6239 6244 6242 6243 6243 6244 6246 6244
6258 6258 6256 6253 6244 6244 6244 6240 6244 6244 6244 6244 6246 6244 6244
6258 6258 6256 6258 6257 6244 6255 6249 6244 6243 6244 6244 6244 6246 6247
6258 6258 6256 6258 6256 6244 6256 6255 6244 6243 6243 6247 6241 6244 6244
6257 6257 6255 6253 6258 6243 6258 6242 6245 6244 6244 6244 6246 6244 6244
6257 6257 6256 6259 6256 6256 6256 6257 6243 6244 6244 6244 6244 6246 6246

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 6242 bytes 100%
1,000 6240 bytes -2 bytes 100%
10,000 6240 bytes 100%
100,000 6239 bytes -1 byte 3.77%
1,000,000 6239 bytes 0.58%
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
6252 bytes +13 bytes (+0.21%)
6252 bytes +13 bytes (+0.21%)
6272 bytes +33 bytes (+0.53%) +20 bytes
6274 bytes +35 bytes (+0.56%) +22 bytes
6302 bytes +63 bytes (+1.01%) +50 bytes
6336 bytes +97 bytes (+1.55%) +84 bytes
6368 bytes +129 bytes (+2.07%) +116 bytes
6405 bytes +166 bytes (+2.66%) +153 bytes
6439 bytes +200 bytes (+3.21%) +187 bytes

Non-DEFLATE Algorithms

Archivers based on completely different compression algorithms often produce superior results.
Unfortunately, browsers only support gzip compression at the moment.
Algorithm Program Parameters Size Compared To Best Zopfli
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 5490 bytes -749 bytes (-12.01%)
Brotli (Wikipedia) brotli brotli -q 11 5843 bytes -396 bytes (-6.35%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 5854 bytes -385 bytes (-6.17%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 6131 bytes -108 bytes (-1.73%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 6239 bytes (0.00%)
ZSTD (Wikipedia) zstd zstd -19 6269 bytes +30 bytes (+0.48%)
LZMA2 (Wikipedia) xz xz -9 6284 bytes +45 bytes (+0.72%)

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 2018.
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