Choose a version:
38% The original file has 18316 bytes (17.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 6892 bytes (6.7k, 38%).

After GZIP compression these minified files vary in size:
Boot
  2477 bytes (2.4k)
CDN
gzip -9
  2251 bytes (2.2k)
local copy
gzip -6 (default)
  2250 bytes (2.2k)
local copy
Baidu
  2236 bytes (2.2k)
CDN
cdnjs
  2236 bytes (2.2k)
CDN
cdnhttps
  2226 bytes (2.2k)
CDN
7zip -mx=9 -tgzip
  2184 bytes (2.1k)
local copy
libdeflate -12
  2184 bytes (2.1k)
local copy
kzip -s0 -rn -b1
  2182 bytes (2.1k)
local copy
pigz -11 -n
  2179 bytes (2.1k)
local copy
Zopfli
  2178 bytes (2.1k)
local copy
Zopfli (defluff)
  2177 bytes (2.1k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.1.min.js

You will automatically get the smallest UnderscoreJS 0.4.1 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 48 bytes by using my UnderscoreJS 0.4.1 Zopfli version instead of the best available CDN (2.20% smaller than cdnhttps, 2178 vs. 2226 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 --mls64 --bsr8 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 64  --mls64
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

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 (2177 bytes).

Verify file integrity

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

MD5:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.4.1/underscore-min.js --location | md5sum
917b0d296f09ba7e360e4648bd91d716  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.1.min.zopfli.js.gz | md5sum
917b0d296f09ba7e360e4648bd91d716  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.4.1/underscore-min.js --location | sha1sum
63b93c2e3fbc194ef03d22588b63d15375455c62  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.1.min.zopfli.js.gz | sha1sum
63b93c2e3fbc194ef03d22588b63d15375455c62  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 2477 bytes 917b0d296f09ba7e360e4648bd91d716 (invalid)
Baidu 2236 bytes 917b0d296f09ba7e360e4648bd91d716 (invalid)
cdnjs 2236 bytes 917b0d296f09ba7e360e4648bd91d716 (invalid)
cdnhttps 2226 bytes 917b0d296f09ba7e360e4648bd91d716 (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
2178 bytes -1 byte zopfli --i10000 --mls64 --bsr8 --lazy --ohh February 2, 2016 @ 23:06
2179 bytes zopfli --i100 --mls32768 --bsr15 --lazy --ohh February 2, 2016 @ 20:57

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

Most recent activity on June 16, 2016 @ 14:33.

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
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178
2178 2178 2178 2179 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178 2178

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 2179 bytes 100%
1,000 2179 bytes 100%
10,000 2178 bytes -1 byte 100%
100,000 2178 bytes 0.29%
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
2182 bytes +4 bytes (+0.18%)
2182 bytes +4 bytes (+0.18%)
2208 bytes +30 bytes (+1.38%) +26 bytes
2234 bytes +56 bytes (+2.57%) +52 bytes
2266 bytes +88 bytes (+4.04%) +84 bytes
2293 bytes +115 bytes (+5.28%) +111 bytes
2313 bytes +135 bytes (+6.20%) +131 bytes
2344 bytes +166 bytes (+7.62%) +162 bytes
2379 bytes +201 bytes (+9.23%) +197 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 1959 bytes -219 bytes (-10.06%)
Brotli (Wikipedia) brotli brotli -q 11 1982 bytes -196 bytes (-9.00%)
ZSTD (Wikipedia) zstd zstd -19 2200 bytes +22 bytes (+1.01%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 2243 bytes +65 bytes (+2.98%)
LZMA2 (Wikipedia) xz xz -9 2264 bytes +86 bytes (+3.95%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 2392 bytes +214 bytes (+9.83%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 2866 bytes +688 bytes (+31.59%)

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