Choose a version:
36% The original file has 15699 bytes (15.3k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 5688 bytes (5.6k, 36%).

After GZIP compression these minified files vary in size:
Boot
  2105 bytes (2.1k)
 CDN
gzip -9
  1927 bytes (1.9k)
 local copy
gzip -6 (default)
  1924 bytes (1.9k)
 local copy
Baidu
  1911 bytes (1.9k)
 CDN
cdnjs
  1911 bytes (1.9k)
 CDN
cdnhttps
  1900 bytes (1.9k)
 CDN
libdeflate -12
  1866 bytes (1.8k)
 local copy
kzip -s0 -rn -b1
  1865 bytes (1.8k)
 local copy
pigz -11 -n
  1863 bytes (1.8k)
 local copy
7zip -mx=9 -tgzip
  1861 bytes (1.8k)
 local copy
Zopfli
  1861 bytes (1.8k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-0.3.2.min.js

You will automatically get the smallest UnderscoreJS 0.3.2 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 39 bytes by using my UnderscoreJS 0.3.2 Zopfli version instead of the best available CDN (2.10% smaller than cdnhttps, 1861 vs. 1900 bytes):
Download
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 --mls32 --bsr7 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 100  --i100
maximum blocks 8  --mb8
maximum length score 32  --mls32
block splitting recursion 7  --bsr7
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/0.3.2/underscore-min.js --location | md5sum
2906bd885102a6cd0b116bf96580b00d  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.3.2.min.zopfli.js.gz | md5sum
2906bd885102a6cd0b116bf96580b00d  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.3.2/underscore-min.js --location | sha1sum
33aa9b1cda80a8c9d7043c987bfa7ee2002ba794  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.3.2.min.zopfli.js.gz | sha1sum
33aa9b1cda80a8c9d7043c987bfa7ee2002ba794  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 2105 bytes 2906bd885102a6cd0b116bf96580b00d (invalid)
Baidu 1911 bytes 2906bd885102a6cd0b116bf96580b00d (invalid)
cdnjs 1911 bytes 2906bd885102a6cd0b116bf96580b00d (invalid)
cdnhttps 1900 bytes 2906bd885102a6cd0b116bf96580b00d (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
1861 bytes – zopfli --i100 --mls32 --bsr7 --lazy --ohh February 2, 2016 @ 20:59

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 @ 14:36.

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
2 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
4 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
6 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
7 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
8 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
9 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
10 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
11 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
12 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
13 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
14 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
15 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
16 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
17 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
18 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
19 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
20 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
21 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
22 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
23 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
25 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
30 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861
40 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861 1861

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 1861 bytes – 100%
1,000 1861 bytes – 100%
10,000 1861 bytes – 100%
100,000 1861 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
0 1865 bytes +4 bytes (+0.21%) –
1 1865 bytes +4 bytes (+0.21%) –
2 1883 bytes +22 bytes (+1.18%) +18 bytes
3 1910 bytes +49 bytes (+2.63%) +45 bytes
4 1945 bytes +84 bytes (+4.51%) +80 bytes
5 1977 bytes +116 bytes (+6.23%) +112 bytes
6 1998 bytes +137 bytes (+7.36%) +133 bytes
7 2032 bytes +171 bytes (+9.19%) +167 bytes
8 2057 bytes +196 bytes (+10.53%) +192 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 1680 bytes -181 bytes (-9.73%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 1708 bytes -153 bytes (-8.22%)
ZSTD (Wikipedia) zstd zstd -19 1891 bytes +30 bytes (+1.61%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 1949 bytes +88 bytes (+4.73%)
LZMA2 (Wikipedia) xz xz -9 1956 bytes +95 bytes (+5.10%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 2012 bytes +151 bytes (+8.11%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 2618 bytes +757 bytes (+40.68%)

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):
Load and Parse Data


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