Choose a version:
36% The original file has 14186 bytes (13.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 5166 bytes (5.0k, 36%).

After GZIP compression these minified files vary in size:
Boot
  1959 bytes (1.9k)
CDN
gzip -6 (default)
  1819 bytes (1.8k)
local copy
gzip -9
  1815 bytes (1.8k)
local copy
Baidu
  1802 bytes (1.8k)
CDN
cdnjs
  1802 bytes (1.8k)
CDN
cdnhttps
  1795 bytes (1.8k)
CDN
libdeflate -12
  1758 bytes (1.7k)
local copy
pigz -11 -n
  1757 bytes (1.7k)
local copy
7zip -mx=9 -tgzip
  1756 bytes (1.7k)
local copy
kzip -s0 -rn -b1
  1755 bytes (1.7k)
local copy
Zopfli
  1753 bytes (1.7k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-0.1.1.min.js

You will automatically get the smallest UnderscoreJS 0.1.1 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 42 bytes by using my UnderscoreJS 0.1.1 Zopfli version instead of the best available CDN (2.40% smaller than cdnhttps, 1753 vs. 1795 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 --i100000 --mb8 --mls32 --bsr6 --lazy --ohh

(found February 3, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 32  --mls32
block splitting recursion 6  --bsr6
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/0.1.1/underscore-min.js --location | md5sum
aca66999b9aac38815facffeb2f861da  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.1.1.min.zopfli.js.gz | md5sum
aca66999b9aac38815facffeb2f861da  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.1.1/underscore-min.js --location | sha1sum
e843c570455a2f6c308f433647ec9a739ac40e0a  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.1.1.min.zopfli.js.gz | sha1sum
e843c570455a2f6c308f433647ec9a739ac40e0a  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 1959 bytes aca66999b9aac38815facffeb2f861da (invalid)
Baidu 1802 bytes aca66999b9aac38815facffeb2f861da (invalid)
cdnjs 1802 bytes aca66999b9aac38815facffeb2f861da (invalid)
cdnhttps 1795 bytes aca66999b9aac38815facffeb2f861da (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
1753 bytes -1 byte zopfli --i100000 --mls32 --bsr6 --lazy --ohh February 3, 2016 @ 07:14
1754 bytes -1 byte zopfli --i10000 --mls32 --bsr6 --lazy --ohh February 2, 2016 @ 23:18
1755 bytes -1 byte zopfli --i1000 --mls1024 --bsr40 --lazy --ohh February 2, 2016 @ 20:57
1756 bytes zopfli --i100 --mls1024 --bsr40 --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 @ 14:40.

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
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1753 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754
1755 1755 1754 1754 1754 1755 1754 1755 1754 1755 1755 1754 1754 1754 1754

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 1756 bytes 100%
1,000 1755 bytes -1 byte 100%
10,000 1754 bytes -1 byte 100%
100,000 1753 bytes -1 byte 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
1755 bytes +2 bytes (+0.11%)
1755 bytes +2 bytes (+0.11%)
1777 bytes +24 bytes (+1.37%) +22 bytes
1815 bytes +62 bytes (+3.54%) +60 bytes
1835 bytes +82 bytes (+4.68%) +80 bytes
1873 bytes +120 bytes (+6.85%) +118 bytes
1891 bytes +138 bytes (+7.87%) +136 bytes
1923 bytes +170 bytes (+9.70%) +168 bytes
1957 bytes +204 bytes (+11.64%) +202 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
Brotli (Wikipedia) brotli brotli -q 11 1590 bytes -163 bytes (-9.30%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 1631 bytes -122 bytes (-6.96%)
ZSTD (Wikipedia) zstd zstd -19 1771 bytes +18 bytes (+1.03%)
LZMA2 (Wikipedia) xz xz -9 1836 bytes +83 bytes (+4.73%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 1857 bytes +104 bytes (+5.93%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 1910 bytes +157 bytes (+8.96%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 2532 bytes +779 bytes (+44.44%)

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