Choose a version:
35% The original file has 25145 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 8686 bytes (8.5k, 35%).

After GZIP compression these minified files vary in size:
Boot
  3196 bytes (3.1k)
CDN, direct link
gzip -6 (default)
  2911 bytes (2.8k)
local copy
Baidu
  2908 bytes (2.8k)
CDN, direct link
gzip -9
  2907 bytes (2.8k)
local copy
unpkg
  2907 bytes (2.8k)
CDN, direct link
cdnjs
  2900 bytes (2.8k)
CDN, direct link
cdnhttps
  2887 bytes (2.8k)
CDN, direct link
libdeflate -12
  2822 bytes (2.8k)
local copy
7zip -mx=9 -tgzip
  2821 bytes (2.8k)
local copy
kzip -s0 -rn -b1
  2819 bytes (2.8k)
local copy
pigz -11 -n
  2816 bytes (2.8k)
local copy
Zopfli
  2816 bytes (2.8k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.1.2.min.js

You will automatically get the smallest UnderscoreJS 1.1.2 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 1.1.2 Zopfli version instead of the best available CDN (2.52% smaller than cdnhttps, 2816 vs. 2887 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 --i100 --mb8 --mls256 --bsr25 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 100  --i100
maximum blocks 8  --mb8
maximum length score 256  --mls256
block splitting recursion 25  --bsr25
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/1.1.2/underscore-min.js --location | md5sum
871e81f985fd52d6d1eb76c46e16cb24  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.1.2.min.zopfli.js.gz | md5sum
871e81f985fd52d6d1eb76c46e16cb24  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.1.2/underscore-min.js --location | sha1sum
bcdd63e18a8a7cf728253f738556ad3b88d74649  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.1.2.min.zopfli.js.gz | sha1sum
bcdd63e18a8a7cf728253f738556ad3b88d74649  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 3196 bytes 871e81f985fd52d6d1eb76c46e16cb24 (invalid)
unpkg 2907 bytes 871e81f985fd52d6d1eb76c46e16cb24 July 11, 2016 @ 16:49
cdnjs 2900 bytes 871e81f985fd52d6d1eb76c46e16cb24 (invalid)
cdnhttps 2887 bytes 871e81f985fd52d6d1eb76c46e16cb24 (invalid)

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 2908 bytes ea1c7faa3017f1d472843766865e58bc 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
2816 bytes zopfli --i100 --mls256 --bsr25 --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
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816
2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816 2816

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 2816 bytes 100%
1,000 2816 bytes 100%
10,000 2816 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
2819 bytes +3 bytes (+0.11%)
2819 bytes +3 bytes (+0.11%)
2841 bytes +25 bytes (+0.89%) +22 bytes
2874 bytes +58 bytes (+2.06%) +55 bytes
2884 bytes +68 bytes (+2.41%) +65 bytes
2916 bytes +100 bytes (+3.55%) +97 bytes
2951 bytes +135 bytes (+4.79%) +132 bytes
2986 bytes +170 bytes (+6.04%) +167 bytes
3019 bytes +203 bytes (+7.21%) +200 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 2544 bytes -272 bytes (-9.66%)
Brotli (Wikipedia) brotli brotli -q 11 2624 bytes -192 bytes (-6.82%)
ZSTD (Wikipedia) zstd zstd -19 2833 bytes +17 bytes (+0.60%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 2844 bytes +28 bytes (+0.99%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 2875 bytes +59 bytes (+2.10%)
LZMA2 (Wikipedia) xz xz -9 2920 bytes +104 bytes (+3.69%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 3427 bytes +611 bytes (+21.70%)

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