Choose a version:
37% The original file has 19960 bytes (19.5k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 7398 bytes (7.2k, 37%).

After GZIP compression these minified files vary in size:
Boot
  2623 bytes (2.6k)
CDN
gzip -6 (default)
  2395 bytes (2.3k)
local copy
gzip -9
  2392 bytes (2.3k)
local copy
Baidu
  2387 bytes (2.3k)
CDN
cdnjs
  2375 bytes (2.3k)
CDN
cdnhttps
  2371 bytes (2.3k)
CDN
7zip -mx=9 -tgzip
  2326 bytes (2.3k)
local copy
libdeflate -12
  2325 bytes (2.3k)
local copy
kzip -s0 -rn -b1
  2322 bytes (2.3k)
local copy
pigz -11 -n
  2320 bytes (2.3k)
local copy
Zopfli
  2318 bytes (2.3k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.6.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 0.4.6 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 53 bytes by using my UnderscoreJS 0.4.6 Zopfli version instead of the best available CDN (2.29% smaller than cdnhttps, 2318 vs. 2371 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 --i1000 --mb8 --mls16 --bsr7 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 1000  --i1000
maximum blocks 8  --mb8
maximum length score 16  --mls16
block splitting recursion 7  --bsr7
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/0.4.6/underscore-min.js --location | md5sum
907b5b4566060b65cda7272b8b65a7f0  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.6.min.zopfli.js.gz | md5sum
907b5b4566060b65cda7272b8b65a7f0  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.4.6/underscore-min.js --location | sha1sum
c254351eda9212b2ae9a999972643d0419a4e408  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.6.min.zopfli.js.gz | sha1sum
c254351eda9212b2ae9a999972643d0419a4e408  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 2623 bytes 907b5b4566060b65cda7272b8b65a7f0 (invalid)
cdnjs 2375 bytes 907b5b4566060b65cda7272b8b65a7f0 (invalid)
cdnhttps 2371 bytes 907b5b4566060b65cda7272b8b65a7f0 (invalid)

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 2387 bytes b9a925b5865fa994e871394287f0e4f7 only whitespaces differ (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
2318 bytes -1 byte zopfli --i1000 --mls16 --bsr7 --lazy --ohh February 2, 2016 @ 20:58
2319 bytes zopfli --i100 --mls32768 --bsr25 --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 @ 14:35.

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 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
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318
2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318 2318

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 2319 bytes 100%
1,000 2318 bytes -1 byte 100%
10,000 2318 bytes 100%
100,000 2318 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
2323 bytes +5 bytes (+0.22%) +1 byte
2322 bytes +4 bytes (+0.17%)
2348 bytes +30 bytes (+1.29%) +26 bytes
2367 bytes +49 bytes (+2.11%) +45 bytes
2393 bytes +75 bytes (+3.24%) +71 bytes
2427 bytes +109 bytes (+4.70%) +105 bytes
2465 bytes +147 bytes (+6.34%) +143 bytes
2486 bytes +168 bytes (+7.25%) +164 bytes
2519 bytes +201 bytes (+8.67%) +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.
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 2091 bytes -227 bytes (-9.79%)
Brotli (Wikipedia) brotli brotli -q 11 2129 bytes -189 bytes (-8.15%)
Zstandard (Wikipedia) zstd zstd -19 2337 bytes +19 bytes (+0.82%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 2393 bytes +75 bytes (+3.24%)
LZMA2 (Wikipedia) xz xz -9 2416 bytes +98 bytes (+4.23%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 2551 bytes +233 bytes (+10.05%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 3005 bytes +687 bytes (+29.64%)

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