Choose a version:
35% The original file has 24686 bytes (24.1k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 8617 bytes (8.4k, 35%).

After GZIP compression these minified files vary in size:
Boot
  3247 bytes (3.2k)
CDN
gzip -6 (default)
  2964 bytes (2.9k)
local copy
Baidu
  2961 bytes (2.9k)
CDN
gzip -9
  2957 bytes (2.9k)
local copy
cdnjs
  2948 bytes (2.9k)
CDN
cdnhttps
  2940 bytes (2.9k)
CDN
libdeflate -12
  2864 bytes (2.8k)
local copy
7zip -mx=9 -tgzip
  2862 bytes (2.8k)
local copy
kzip -s0 -rn -b0
  2861 bytes (2.8k)
local copy
pigz -11 -n
  2860 bytes (2.8k)
local copy
Zopfli
  2858 bytes (2.8k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.0.0.min.js

You will automatically get the smallest UnderscoreJS 1.0.0 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 82 bytes by using my UnderscoreJS 1.0.0 Zopfli version instead of the best available CDN (2.87% smaller than cdnhttps, 2858 vs. 2940 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 --mls8192 --bsr23 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 1000  --i1000
maximum blocks 8  --mb8
maximum length score 8192  --mls8192
block splitting recursion 23  --bsr23
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/1.0.0/underscore-min.js --location | md5sum
da836e0cccc9e03205b48ffe4c509411  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.0.0.min.zopfli.js.gz | md5sum
da836e0cccc9e03205b48ffe4c509411  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.0.0/underscore-min.js --location | sha1sum
ad0775c89a87111019840dc7bbf902d832e4ccdf  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.0.0.min.zopfli.js.gz | sha1sum
ad0775c89a87111019840dc7bbf902d832e4ccdf  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 3247 bytes da836e0cccc9e03205b48ffe4c509411 (invalid)
cdnjs 2948 bytes da836e0cccc9e03205b48ffe4c509411 (invalid)
cdnhttps 2940 bytes da836e0cccc9e03205b48ffe4c509411 (invalid)

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 2961 bytes dd705e1f9d996a49f40f6ce161d75e65 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:

1.9.0,
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
2858 bytes -1 byte zopfli --i1000 --mls8192 --bsr23 --lazy --ohh February 2, 2016 @ 19:07
2859 bytes zopfli --i100 --mls8192 --bsr23 --lazy --ohh February 2, 2016 @ 19:07

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 @ 14:39.

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.
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
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858
2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858 2858

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 2859 bytes 100%
1,000 2858 bytes -1 byte 100%
10,000 2858 bytes 100%
100,000
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
2861 bytes +3 bytes (+0.10%)
2861 bytes +3 bytes (+0.10%)
2892 bytes +34 bytes (+1.19%) +31 bytes
2926 bytes +68 bytes (+2.38%) +65 bytes
2939 bytes +81 bytes (+2.83%) +78 bytes
2970 bytes +112 bytes (+3.92%) +109 bytes
3004 bytes +146 bytes (+5.11%) +143 bytes
3039 bytes +181 bytes (+6.33%) +178 bytes
3073 bytes +215 bytes (+7.52%) +212 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 2572 bytes -286 bytes (-10.01%)
Brotli (Wikipedia) brotli brotli -q 11 2657 bytes -201 bytes (-7.03%)
ZSTD (Wikipedia) zstd zstd -19 2876 bytes +18 bytes (+0.63%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 2900 bytes +42 bytes (+1.47%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 2912 bytes +54 bytes (+1.89%)
LZMA2 (Wikipedia) xz xz -9 2948 bytes +90 bytes (+3.15%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 3453 bytes +595 bytes (+20.82%)

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