Choose a version:
35% The original file has 25230 bytes (24.6k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 8758 bytes (8.6k, 35%).

After GZIP compression these minified files vary in size:
Boot
  3247 bytes (3.2k)
CDN, direct link
Baidu
  2945 bytes (2.9k)
CDN, direct link
gzip -6 (default)
  2945 bytes (2.9k)
local copy
gzip -9
  2939 bytes (2.9k)
local copy
unpkg
  2935 bytes (2.9k)
CDN, direct link
cdnjs
  2933 bytes (2.9k)
CDN, direct link
cdnhttps
  2921 bytes (2.9k)
CDN, direct link
libdeflate -12
  2856 bytes (2.8k)
local copy
7zip -mx=9 -tgzip
  2854 bytes (2.8k)
local copy
kzip -s0 -rn -b0
  2853 bytes (2.8k)
local copy
pigz -11 -n
  2851 bytes (2.8k)
local copy
Zopfli
  2850 bytes (2.8k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.1.1.min.js

You will automatically get the smallest UnderscoreJS 1.1.1 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 71 bytes by using my UnderscoreJS 1.1.1 Zopfli version instead of the best available CDN (2.49% smaller than cdnhttps, 2850 vs. 2921 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 --mls512 --bsr13 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 1000  --i1000
maximum blocks 8  --mb8
maximum length score 512  --mls512
block splitting recursion 13  --bsr13
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/1.1.1/underscore-min.js --location | md5sum
f8b8f0bbdb1570777f5274c70eaf1414  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.1.1.min.zopfli.js.gz | md5sum
f8b8f0bbdb1570777f5274c70eaf1414  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.1.1/underscore-min.js --location | sha1sum
5de53da57fc8b7ab6c77d597c22875d747352fef  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.1.1.min.zopfli.js.gz | sha1sum
5de53da57fc8b7ab6c77d597c22875d747352fef  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 3247 bytes f8b8f0bbdb1570777f5274c70eaf1414 (invalid)
unpkg 2935 bytes f8b8f0bbdb1570777f5274c70eaf1414 July 11, 2016 @ 16:49
cdnjs 2933 bytes f8b8f0bbdb1570777f5274c70eaf1414 (invalid)
cdnhttps 2921 bytes f8b8f0bbdb1570777f5274c70eaf1414 (invalid)

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 2945 bytes 0a41b3c224b1c9a256d0a815402dd1ba 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:

1.8.3, 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
2850 bytes -1 byte zopfli --i1000 --mls512 --bsr13 --lazy --ohh February 2, 2016 @ 19:50
2851 bytes zopfli --i100 --mls16384 --bsr7 --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 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
2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2858 2858 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2858 2858 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2859 2859 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2859 2859 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2859 2859 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2858 2858 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2858 2858 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2860 2860 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2858 2858 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2858 2858 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2858 2858 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2858 2858 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2858 2858 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2858 2858 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2859 2859 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2859 2859 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2859 2859 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2859 2859 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850
2858 2858 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850 2850

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 2851 bytes 100%
1,000 2850 bytes -1 byte 100%
10,000 2850 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
2853 bytes +3 bytes (+0.11%)
2853 bytes +3 bytes (+0.11%)
2877 bytes +27 bytes (+0.95%) +24 bytes
2906 bytes +56 bytes (+1.96%) +53 bytes
2916 bytes +66 bytes (+2.32%) +63 bytes
2950 bytes +100 bytes (+3.51%) +97 bytes
2982 bytes +132 bytes (+4.63%) +129 bytes
3016 bytes +166 bytes (+5.82%) +163 bytes
3052 bytes +202 bytes (+7.09%) +199 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 2564 bytes -286 bytes (-10.04%)
Brotli (Wikipedia) brotli brotli -q 11 2648 bytes -202 bytes (-7.09%)
ZSTD (Wikipedia) zstd zstd -19 2859 bytes +9 bytes (+0.32%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 2867 bytes +17 bytes (+0.60%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 2910 bytes +60 bytes (+2.11%)
LZMA2 (Wikipedia) xz xz -9 2944 bytes +94 bytes (+3.30%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 3445 bytes +595 bytes (+20.88%)

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 2017.
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