Choose a version:
36% The original file has 13872 bytes (13.5k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 5033 bytes (4.9k, 36%).

After GZIP compression these minified files vary in size:
Boot
  1899 bytes (1.9k)
CDN, direct link
gzip -6 (default)
  1760 bytes (1.7k)
local copy
gzip -9
  1756 bytes (1.7k)
local copy
Baidu
  1743 bytes (1.7k)
CDN, direct link
cdnjs
  1743 bytes (1.7k)
CDN, direct link
cdnhttps
  1736 bytes (1.7k)
CDN, direct link
libdeflate -12
  1699 bytes (1.7k)
local copy
pigz -11 -n
  1697 bytes (1.7k)
local copy
7zip -mx=9 -tgzip
  1696 bytes (1.7k)
local copy
kzip -s0 -rn -b1
  1696 bytes (1.7k)
local copy
Zopfli
  1694 bytes (1.7k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-0.1.0.min.js

You will automatically get the smallest UnderscoreJS 0.1.0 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 42 bytes by using my UnderscoreJS 0.1.0 Zopfli version instead of the best available CDN (2.48% smaller than cdnhttps, 1694 vs. 1736 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 --mls2 --bsr8 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 2  --mls2
block splitting recursion 8  --bsr8
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/0.1.0/underscore-min.js --location | md5sum
05d6dea85a8d7c67efb029754d71f964  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.1.0.min.zopfli.js.gz | md5sum
05d6dea85a8d7c67efb029754d71f964  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.1.0/underscore-min.js --location | sha1sum
c5c2f6c9a5150ff998abd530a553bf8779df6d04  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.1.0.min.zopfli.js.gz | sha1sum
c5c2f6c9a5150ff998abd530a553bf8779df6d04  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 1899 bytes 05d6dea85a8d7c67efb029754d71f964 (invalid)
Baidu 1743 bytes 05d6dea85a8d7c67efb029754d71f964 (invalid)
cdnjs 1743 bytes 05d6dea85a8d7c67efb029754d71f964 (invalid)
cdnhttps 1736 bytes 05d6dea85a8d7c67efb029754d71f964 (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
1694 bytes -2 bytes zopfli --i10000 --mls2 --bsr8 --lazy --ohh February 2, 2016 @ 23:24
1696 bytes zopfli --i100 --mls32 --bsr18 --lazy --ohh February 2, 2016 @ 20:58

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
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694
1694 1694 1694 1695 1694 1694 1694 1695 1695 1695 1694 1694 1694 1694 1694

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 1696 bytes 100%
1,000 1696 bytes 100%
10,000 1694 bytes -2 bytes 100%
100,000 1694 bytes 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
1696 bytes +2 bytes (+0.12%)
1696 bytes +2 bytes (+0.12%)
1716 bytes +22 bytes (+1.30%) +20 bytes
1750 bytes +56 bytes (+3.31%) +54 bytes
1785 bytes +91 bytes (+5.37%) +89 bytes
1812 bytes +118 bytes (+6.97%) +116 bytes
1841 bytes +147 bytes (+8.68%) +145 bytes
1877 bytes +183 bytes (+10.80%) +181 bytes
1912 bytes +218 bytes (+12.87%) +216 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 1547 bytes -147 bytes (-8.68%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 1576 bytes -118 bytes (-6.97%)
ZSTD (Wikipedia) zstd zstd -19 1711 bytes +17 bytes (+1.00%)
LZMA2 (Wikipedia) xz xz -9 1780 bytes +86 bytes (+5.08%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 1795 bytes +101 bytes (+5.96%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 1832 bytes +138 bytes (+8.15%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 2487 bytes +793 bytes (+46.81%)

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