Choose a version:
37% The original file has 17709 bytes (17.3k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 6494 bytes (6.3k, 37%).

After GZIP compression these minified files vary in size:
Boot
  2372 bytes (2.3k)
CDN, direct link
gzip -9
  2158 bytes (2.1k)
local copy
gzip -6 (default)
  2157 bytes (2.1k)
local copy
Baidu
  2145 bytes (2.1k)
CDN, direct link
cdnjs
  2145 bytes (2.1k)
CDN, direct link
cdnhttps
  2133 bytes (2.1k)
CDN, direct link
libdeflate -12
  2092 bytes (2.0k)
local copy
kzip -s0 -rn -b1
  2090 bytes (2.0k)
local copy
7zip -mx=9 -tgzip
  2088 bytes (2.0k)
local copy
pigz -11 -n
  2088 bytes (2.0k)
local copy
Zopfli
  2087 bytes (2.0k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.0.min.js

You will automatically get the smallest UnderscoreJS 0.4.0 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 46 bytes by using my UnderscoreJS 0.4.0 Zopfli version instead of the best available CDN (2.20% smaller than cdnhttps, 2087 vs. 2133 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 --mls16384 --bsr23 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 1000  --i1000
maximum blocks 8  --mb8
maximum length score 16384  --mls16384
block splitting recursion 23  --bsr23
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/0.4.0/underscore-min.js --location | md5sum
a45d849bcc6d5d3d29e235e4c83528a1  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.0.min.zopfli.js.gz | md5sum
a45d849bcc6d5d3d29e235e4c83528a1  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.4.0/underscore-min.js --location | sha1sum
efe8ae9b678eb2e4707d9c66b29b97bd48e7e8c9  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.0.min.zopfli.js.gz | sha1sum
efe8ae9b678eb2e4707d9c66b29b97bd48e7e8c9  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 2372 bytes a45d849bcc6d5d3d29e235e4c83528a1 (invalid)
Baidu 2145 bytes a45d849bcc6d5d3d29e235e4c83528a1 (invalid)
cdnjs 2145 bytes a45d849bcc6d5d3d29e235e4c83528a1 (invalid)
cdnhttps 2133 bytes a45d849bcc6d5d3d29e235e4c83528a1 (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
2087 bytes -1 byte zopfli --i1000 --mls16384 --bsr23 --lazy --ohh February 2, 2016 @ 21:00
2088 bytes zopfli --i100 --mls8192 --bsr14 --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 @ 14:40.

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
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087
2087 2087 2087 2087 2087 2088 2087 2087 2087 2087 2087 2087 2087 2087 2087

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 2088 bytes 100%
1,000 2087 bytes -1 byte 100%
10,000 2087 bytes 100%
100,000 2087 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
2090 bytes +3 bytes (+0.14%)
2090 bytes +3 bytes (+0.14%)
2116 bytes +29 bytes (+1.39%) +26 bytes
2137 bytes +50 bytes (+2.40%) +47 bytes
2170 bytes +83 bytes (+3.98%) +80 bytes
2196 bytes +109 bytes (+5.22%) +106 bytes
2229 bytes +142 bytes (+6.80%) +139 bytes
2255 bytes +168 bytes (+8.05%) +165 bytes
2291 bytes +204 bytes (+9.77%) +201 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 1889 bytes -198 bytes (-9.49%)
Brotli (Wikipedia) brotli brotli -q 11 1893 bytes -194 bytes (-9.30%)
ZSTD (Wikipedia) zstd zstd -19 2115 bytes +28 bytes (+1.34%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 2152 bytes +65 bytes (+3.11%)
LZMA2 (Wikipedia) xz xz -9 2172 bytes +85 bytes (+4.07%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 2256 bytes +169 bytes (+8.10%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 2788 bytes +701 bytes (+33.59%)

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