Choose a version:
36% The original file has 16400 bytes (16.0k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 5982 bytes (5.8k, 36%).

After GZIP compression these minified files vary in size:
Boot
  2192 bytes (2.1k)
CDN
gzip -6 (default)
  2002 bytes (2.0k)
local copy
gzip -9
  2002 bytes (2.0k)
local copy
Baidu
  1985 bytes (1.9k)
CDN
cdnjs
  1985 bytes (1.9k)
CDN
cdnhttps
  1978 bytes (1.9k)
CDN
libdeflate -12
  1941 bytes (1.9k)
local copy
kzip -s0 -rn -b1
  1938 bytes (1.9k)
local copy
pigz -11 -n
  1938 bytes (1.9k)
local copy
7zip -mx=9 -tgzip
  1937 bytes (1.9k)
local copy
Zopfli
  1935 bytes (1.9k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-0.3.3.min.js

You will automatically get the smallest UnderscoreJS 0.3.3 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 43 bytes by using my UnderscoreJS 0.3.3 Zopfli version instead of the best available CDN (2.22% smaller than cdnhttps, 1935 vs. 1978 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 --bsr2 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 1000  --i1000
maximum blocks 8  --mb8
maximum length score 512  --mls512
block splitting recursion 2  --bsr2
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/0.3.3/underscore-min.js --location | md5sum
1e05fc11b907519a5c1d6546cc036217  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.3.3.min.zopfli.js.gz | md5sum
1e05fc11b907519a5c1d6546cc036217  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.3.3/underscore-min.js --location | sha1sum
db5bd7f83bb9e0828832bf45f68509443263141f  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.3.3.min.zopfli.js.gz | sha1sum
db5bd7f83bb9e0828832bf45f68509443263141f  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 2192 bytes 1e05fc11b907519a5c1d6546cc036217 (invalid)
Baidu 1985 bytes 1e05fc11b907519a5c1d6546cc036217 (invalid)
cdnjs 1985 bytes 1e05fc11b907519a5c1d6546cc036217 (invalid)
cdnhttps 1978 bytes 1e05fc11b907519a5c1d6546cc036217 (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
1935 bytes -2 bytes zopfli --i1000 --mls512 --bsr2 --lazy --ohh February 2, 2016 @ 20:59
1937 bytes zopfli --i100 --mls128 --bsr7 --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
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935
1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935 1935

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 1936 bytes 100%
1,000 1935 bytes -1 byte 100%
10,000 1935 bytes 100%
100,000 1935 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
1938 bytes +3 bytes (+0.16%)
1938 bytes +3 bytes (+0.16%)
1960 bytes +25 bytes (+1.29%) +22 bytes
1985 bytes +50 bytes (+2.58%) +47 bytes
2017 bytes +82 bytes (+4.24%) +79 bytes
2052 bytes +117 bytes (+6.05%) +114 bytes
2074 bytes +139 bytes (+7.18%) +136 bytes
2098 bytes +163 bytes (+8.42%) +160 bytes
2132 bytes +197 bytes (+10.18%) +194 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 1744 bytes -191 bytes (-9.87%)
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 1768 bytes -167 bytes (-8.63%)
ZSTD (Wikipedia) zstd zstd -19 1965 bytes +30 bytes (+1.55%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 2023 bytes +88 bytes (+4.55%)
LZMA2 (Wikipedia) xz xz -9 2028 bytes +93 bytes (+4.81%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 2063 bytes +128 bytes (+6.61%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 2675 bytes +740 bytes (+38.24%)

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