Choose a version:
35% The original file has 34097 bytes (33.3k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 11887 bytes (11.6k, 35%).

After GZIP compression these minified files vary in size:
Boot
  4445 bytes (4.3k)
CDN
Baidu
  3999 bytes (3.9k)
CDN
gzip -6 (default)
  3986 bytes (3.9k)
local copy
cdnjs
  3980 bytes (3.9k)
CDN
gzip -9
  3980 bytes (3.9k)
local copy
unpkg
  3967 bytes (3.9k)
CDN
cdnhttps
  3962 bytes (3.9k)
CDN
libdeflate -12
  3890 bytes (3.8k)
local copy
7zip -mx=9 -tgzip
  3887 bytes (3.8k)
local copy
kzip -s0 -rn -b1
  3884 bytes (3.8k)
local copy
pigz -11 -n
  3874 bytes (3.8k)
local copy
Zopfli
  3866 bytes (3.8k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.2.1.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 1.2.1 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 96 bytes by using my UnderscoreJS 1.2.1 Zopfli version instead of the best available CDN (2.48% smaller than cdnhttps, 3866 vs. 3962 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 --mls512 --bsr12 --lazy --ohh

(found September 8, 2015)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 512  --mls512
block splitting recursion 12  --bsr12
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/1.2.1/underscore-min.js --location | md5sum
08d02bde090fcd9ad5e7587838e252d7  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.2.1.min.zopfli.js.gz | md5sum
08d02bde090fcd9ad5e7587838e252d7  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.2.1/underscore-min.js --location | sha1sum
083bbcd739b7b1336f13c72603e1704b164c02bd  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.2.1.min.zopfli.js.gz | sha1sum
083bbcd739b7b1336f13c72603e1704b164c02bd  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 4445 bytes 08d02bde090fcd9ad5e7587838e252d7 (invalid)
cdnjs 3980 bytes 08d02bde090fcd9ad5e7587838e252d7 (invalid)
unpkg 3967 bytes 08d02bde090fcd9ad5e7587838e252d7 July 11, 2016 @ 16:49
cdnhttps 3962 bytes 08d02bde090fcd9ad5e7587838e252d7 December 24, 2015 @ 07:36

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 3999 bytes e6ac0afe8f060689dfe428a4b5965f01 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
3866 bytes -1 byte zopfli --i10000 --mls512 --bsr12 --lazy --ohh September 8, 2015 @ 16:09
3867 bytes -1 byte zopfli --i1000 --mls512 --bsr11 --lazy --ohh September 8, 2015 @ 10:59
3868 bytes zopfli --i100 --mls512 --bsr10 --lazy --ohh September 8, 2015 @ 10:32

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

Most recent activity on July 11, 2016 @ 16:49.

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
3875 3875 3875 3875 3874 3875 3876 3876 3876 3877 3875 3873 3873 3872 3872
3869 3869 3866 3867 3869 3868 3866 3868 3867 3868 3867 3870 3869 3869 3869
3868 3868 3868 3870 3869 3869 3869 3870 3866 3867 3870 3868 3869 3870 3870
3868 3868 3867 3868 3869 3869 3866 3867 3866 3868 3872 3870 3869 3869 3869
3870 3870 3869 3867 3869 3868 3867 3866 3867 3868 3870 3870 3868 3868 3868
3870 3870 3868 3867 3867 3867 3868 3869 3867 3868 3867 3867 3869 3869 3869
3870 3870 3867 3869 3869 3868 3866 3870 3866 3868 3870 3870 3868 3868 3868
3867 3867 3867 3867 3869 3869 3866 3866 3866 3870 3873 3868 3869 3869 3869
3870 3870 3867 3867 3869 3869 3868 3868 3866 3870 3873 3868 3869 3869 3869
3868 3868 3867 3867 3869 3869 3866 3867 3867 3870 3869 3870 3868 3868 3868
3870 3870 3869 3869 3869 3868 3867 3867 3867 3869 3870 3868 3869 3868 3868
3867 3867 3867 3869 3869 3869 3866 3867 3867 3869 3873 3870 3869 3869 3869
3870 3870 3869 3867 3869 3869 3866 3867 3866 3868 3868 3868 3868 3868 3868
3867 3867 3869 3867 3869 3869 3866 3870 3867 3868 3868 3870 3869 3869 3869
3870 3870 3869 3868 3869 3869 3866 3868 3867 3867 3872 3868 3868 3868 3868
3870 3870 3869 3867 3869 3869 3868 3870 3867 3868 3867 3867 3868 3868 3868
3867 3867 3869 3867 3869 3869 3866 3866 3866 3870 3873 3867 3868 3868 3868
3870 3870 3869 3867 3869 3869 3866 3870 3866 3870 3868 3867 3870 3868 3868
3870 3870 3869 3867 3869 3869 3867 3866 3867 3870 3869 3877 3869 3868 3868
3869 3869 3867 3867 3869 3869 3866 3867 3866 3869 3872 3867 3869 3868 3868
3867 3867 3869 3867 3869 3868 3870 3868 3867 3869 3870 3867 3868 3868 3868
3870 3870 3869 3867 3869 3869 3866 3870 3867 3869 3873 3868 3869 3868 3868
3870 3870 3869 3867 3869 3869 3867 3867 3867 3868 3869 3869 3868 3868 3868

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 3868 bytes 100%
1,000 3867 bytes -1 byte 100%
10,000 3866 bytes -1 byte 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
3884 bytes +18 bytes (+0.47%)
3884 bytes +18 bytes (+0.47%)
3899 bytes +33 bytes (+0.85%) +15 bytes
3894 bytes +28 bytes (+0.72%) +10 bytes
3918 bytes +52 bytes (+1.35%) +34 bytes
3942 bytes +76 bytes (+1.97%) +58 bytes
3974 bytes +108 bytes (+2.79%) +90 bytes
4008 bytes +142 bytes (+3.67%) +124 bytes
4041 bytes +175 bytes (+4.53%) +157 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 3480 bytes -386 bytes (-9.98%)
Brotli (Wikipedia) brotli brotli -q 11 3561 bytes -305 bytes (-7.89%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 3853 bytes -13 bytes (-0.34%)
Zstandard (Wikipedia) zstd zstd -19 3901 bytes +35 bytes (+0.91%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 3934 bytes +68 bytes (+1.76%)
LZMA2 (Wikipedia) xz xz -9 3964 bytes +98 bytes (+2.53%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 4330 bytes +464 bytes (+12.00%)

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