Choose a version:
31% The original file has 58233 bytes (56.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 18042 bytes (17.6k, 31%).

After GZIP compression these minified files vary in size:
Boot
  7236 bytes (7.1k)
CDN
gzip -6 (default)
  6450 bytes (6.3k)
local copy
gzip -9
  6442 bytes (6.3k)
local copy
cdnjs
  6418 bytes (6.3k)
CDN
unpkg
  6418 bytes (6.3k)
CDN
7zip -mx=9 -tgzip
  6252 bytes (6.1k)
local copy
libdeflate -12
  6245 bytes (6.1k)
local copy
pigz -11 -n
  6243 bytes (6.1k)
local copy
kzip -s0 -rn -b1
  6241 bytes (6.1k)
local copy
Zopfli
  6231 bytes (6.1k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.9.0.min.js

You will automatically get the smallest UnderscoreJS 1.9.0 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 187 bytes by using my UnderscoreJS 1.9.0 Zopfli version instead of the best available CDN (3.00% smaller than unpkg, 6231 vs. 6418 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 --mls4096 --bsr18 --lazy --ohh

(found April 23, 2018)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 4096  --mls4096
block splitting recursion 18  --bsr18
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/1.9.0/underscore-min.js --location | md5sum
8926a01daa0f08e58e7f19f5e7049a2d  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.9.0.min.zopfli.js.gz | md5sum
8926a01daa0f08e58e7f19f5e7049a2d  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.9.0/underscore-min.js --location | sha1sum
43ff9826cd320a8b680f364c7a8adf3da81cf571  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.9.0.min.zopfli.js.gz | sha1sum
43ff9826cd320a8b680f364c7a8adf3da81cf571  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 7236 bytes 8926a01daa0f08e58e7f19f5e7049a2d (invalid)
cdnjs 6418 bytes 8926a01daa0f08e58e7f19f5e7049a2d (invalid)
unpkg 6418 bytes 8926a01daa0f08e58e7f19f5e7049a2d (invalid)

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

Other Versions

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

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
6231 bytes -1 byte zopfli --i10000 --mls4096 --bsr18 --lazy --ohh April 23, 2018 @ 10:45
6232 bytes -1 byte zopfli --i10000 --mls256 --bsr20 --lazy --ohh April 23, 2018 @ 10:29
6233 bytes zopfli --i100 --mls512 --bsr14 --lazy --ohh April 23, 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 April 23, 2018 @ 20:57.

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
6239 6239 6247 6240 6246 6241 6241 6241 6241 6241 6241 6236 6239 6239 6239
6247 6247 6245 6247 6247 6246 6246 6241 6233 6232 6241 6237 6236 6236 6238
6248 6248 6246 6247 6247 6243 6249 6235 6238 6239 6235 6234 6235 6236 6236
6247 6247 6246 6247 6249 6243 6246 6233 6235 6236 6234 6234 6234 6236 6236
6240 6240 6243 6240 6247 6243 6246 6234 6233 6233 6234 6235 6236 6236 6236
6245 6245 6246 6246 6247 6245 6246 6241 6233 6235 6241 6231 6237 6238 6236
6246 6246 6247 6247 6247 6243 6246 6232 6233 6235 6242 6235 6237 6235 6235
6239 6239 6247 6247 6246 6236 6235 6235 6235 6234 6235 6234 6236 6238 6239
6246 6246 6247 6247 6247 6246 6246 6233 6235 6234 6241 6236 6236 6236 6240
6244 6244 6247 6247 6246 6249 6247 6232 6235 6235 6234 6234 6236 6236 6236
6247 6247 6247 6246 6233 6243 6233 6233 6232 6231 6234 6234 6233 6236 6236
6247 6247 6247 6247 6247 6243 6246 6234 6235 6235 6234 6234 6233 6236 6236
6244 6244 6246 6246 6249 6245 6246 6233 6232 6236 6231 6234 6236 6237 6236
6247 6247 6247 6247 6247 6247 6246 6234 6235 6235 6234 6234 6237 6238 6239
6246 6246 6247 6247 6247 6243 6246 6246 6232 6234 6241 6231 6236 6238 6236
6246 6246 6247 6245 6246 6235 6234 6235 6236 6232 6241 6236 6234 6236 6236
6246 6246 6247 6247 6247 6247 6246 6232 6235 6239 6234 6234 6235 6238 6239
6246 6246 6247 6247 6247 6243 6247 6234 6233 6235 6241 6235 6234 6236 6236
6245 6245 6247 6248 6247 6243 6246 6232 6233 6239 6234 6235 6233 6238 6239
6243 6243 6247 6247 6247 6244 6247 6247 6238 6239 6235 6236 6237 6236 6236
6246 6246 6247 6247 6247 6247 6246 6235 6235 6235 6234 6235 6231 6236 6236
6247 6247 6246 6247 6248 6244 6246 6234 6231 6234 6242 6234 6235 6236 6236
6246 6246 6246 6247 6246 6243 6246 6233 6233 6239 6241 6234 6236 6239 6236

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 6233 bytes 100%
1,000 6233 bytes 100%
10,000 6231 bytes -2 bytes 100%
100,000 6231 bytes 4.35%
1,000,000 6231 bytes 1.74%
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
6242 bytes +11 bytes (+0.18%) +1 byte
6241 bytes +10 bytes (+0.16%)
6264 bytes +33 bytes (+0.53%) +23 bytes
6265 bytes +34 bytes (+0.55%) +24 bytes
6291 bytes +60 bytes (+0.96%) +50 bytes
6325 bytes +94 bytes (+1.51%) +84 bytes
6360 bytes +129 bytes (+2.07%) +119 bytes
6396 bytes +165 bytes (+2.65%) +155 bytes
6435 bytes +204 bytes (+3.27%) +194 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 5480 bytes -751 bytes (-12.05%)
Brotli (Wikipedia) brotli brotli -q 11 5825 bytes -406 bytes (-6.52%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 5845 bytes -386 bytes (-6.19%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 6113 bytes -118 bytes (-1.89%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 6236 bytes +5 bytes (+0.08%)
ZSTD (Wikipedia) zstd zstd -19 6262 bytes +31 bytes (+0.50%)
LZMA2 (Wikipedia) xz xz -9 6272 bytes +41 bytes (+0.66%)

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