Choose a version:
37% The original file has 15304 bytes (14.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 5633 bytes (5.5k, 37%).

After GZIP compression these minified files vary in size:
Boot
  2096 bytes (2.0k)
CDN
gzip -6 (default)
  1910 bytes (1.9k)
local copy
gzip -9
  1909 bytes (1.9k)
local copy
Baidu
  1896 bytes (1.9k)
CDN
cdnjs
  1896 bytes (1.9k)
CDN
cdnhttps
  1886 bytes (1.8k)
CDN
libdeflate -12
  1855 bytes (1.8k)
local copy
pigz -11 -n
  1854 bytes (1.8k)
local copy
kzip -s0 -rn -b0
  1853 bytes (1.8k)
local copy
7zip -mx=9 -tgzip
  1851 bytes (1.8k)
local copy
Zopfli
  1851 bytes (1.8k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-0.2.0.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 0.2.0 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 35 bytes by using my UnderscoreJS 0.2.0 Zopfli version instead of the best available CDN (1.89% smaller than cdnhttps, 1851 vs. 1886 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/0.2.0/underscore-min.js --location | md5sum
dc33978fc14cc9dc4074e32a95bdad4e  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-0.2.0.min.zopfli.js.gz | md5sum
dc33978fc14cc9dc4074e32a95bdad4e  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.2.0/underscore-min.js --location | sha1sum
e2175ab6d89def7f267558e74839a2d9728d0bc7  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-0.2.0.min.zopfli.js.gz | sha1sum
e2175ab6d89def7f267558e74839a2d9728d0bc7  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 2096 bytes dc33978fc14cc9dc4074e32a95bdad4e (invalid)
Baidu 1896 bytes dc33978fc14cc9dc4074e32a95bdad4e (invalid)
cdnjs 1896 bytes dc33978fc14cc9dc4074e32a95bdad4e (invalid)
cdnhttps 1886 bytes dc33978fc14cc9dc4074e32a95bdad4e (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
1851 bytes -1 byte zopfli --i10000 --mls2 --bsr8 --lazy --ohh February 2, 2016 @ 23:37
1852 bytes -1 byte zopfli --i1000 --mls32 --bsr25 --lazy --ohh February 2, 2016 @ 20:58
1853 bytes zopfli --i100 --mls32 --bsr25 --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 @ 14:38.

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
1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851
1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851
1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851
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1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851
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1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851
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1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851
1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851
1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851 1851

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 1852 bytes 100%
1,000 1852 bytes 100%
10,000 1851 bytes -1 byte 100%
100,000 1851 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
1853 bytes +2 bytes (+0.11%)
1853 bytes +2 bytes (+0.11%)
1870 bytes +19 bytes (+1.03%) +17 bytes
1903 bytes +52 bytes (+2.81%) +50 bytes
1938 bytes +87 bytes (+4.70%) +85 bytes
1963 bytes +112 bytes (+6.05%) +110 bytes
1995 bytes +144 bytes (+7.78%) +142 bytes
2029 bytes +178 bytes (+9.62%) +176 bytes
2057 bytes +206 bytes (+11.13%) +204 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
Brotli (Wikipedia) brotli brotli -q 11 1683 bytes -168 bytes (-9.08%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 1684 bytes -167 bytes (-9.02%)
Zstandard (Wikipedia) zstd zstd -19 1871 bytes +20 bytes (+1.08%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 1922 bytes +71 bytes (+3.84%)
LZMA2 (Wikipedia) xz xz -9 1936 bytes +85 bytes (+4.59%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 1961 bytes +110 bytes (+5.94%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 2602 bytes +751 bytes (+40.57%)

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