Choose a version:
35% The original file has 26210 bytes (25.6k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 9304 bytes (9.1k, 35%).

After GZIP compression these minified files vary in size:
Boot
  3545 bytes (3.5k)
CDN
Baidu
  3227 bytes (3.2k)
CDN
gzip -6 (default)
  3226 bytes (3.2k)
local copy
gzip -9
  3223 bytes (3.1k)
local copy
unpkg
  3220 bytes (3.1k)
CDN
cdnjs
  3213 bytes (3.1k)
CDN
cdnhttps
  3202 bytes (3.1k)
CDN
libdeflate -12
  3137 bytes (3.1k)
local copy
7zip -mx=9 -tgzip
  3133 bytes (3.1k)
local copy
kzip -s0 -rn -b1
  3131 bytes (3.1k)
local copy
pigz -11 -n
  3124 bytes (3.1k)
local copy
Zopfli
  3120 bytes (3.0k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.1.3.min.js

You will automatically get the smallest UnderscoreJS 1.1.3 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 82 bytes by using my UnderscoreJS 1.1.3 Zopfli version instead of the best available CDN (2.63% smaller than cdnhttps, 3120 vs. 3202 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 --mls64 --bsr22 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 1000  --i1000
maximum blocks 8  --mb8
maximum length score 64  --mls64
block splitting recursion 22  --bsr22
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/1.1.3/underscore-min.js --location | md5sum
e9ac5a3abb3cdd20049c4f875e9e4c33  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.1.3.min.zopfli.js.gz | md5sum
e9ac5a3abb3cdd20049c4f875e9e4c33  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.1.3/underscore-min.js --location | sha1sum
9e389ede8ed7f782c5ecf2f87c3afa3b1e309bff  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.1.3.min.zopfli.js.gz | sha1sum
9e389ede8ed7f782c5ecf2f87c3afa3b1e309bff  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 3545 bytes e9ac5a3abb3cdd20049c4f875e9e4c33 (invalid)
unpkg 3220 bytes e9ac5a3abb3cdd20049c4f875e9e4c33 July 11, 2016 @ 16:49
cdnjs 3213 bytes e9ac5a3abb3cdd20049c4f875e9e4c33 (invalid)
cdnhttps 3202 bytes e9ac5a3abb3cdd20049c4f875e9e4c33 (invalid)

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 3227 bytes 5ac3f12750c8d9fe39f426f37504c90a only whitespaces differ (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
3120 bytes -1 byte zopfli --i1000 --mls64 --bsr22 --lazy --ohh February 2, 2016 @ 19:46
3121 bytes zopfli --i100 --mls8 --bsr18 --lazy --ohh February 2, 2016 @ 19:07

If there are multiple parameter sets yielding the same compressed size, only the first one found is shown.

Most recent activity on July 11, 2016 @ 16:49.

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.
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
3122 3122 3122 3121 3122 3122 3122 3121 3121 3121 3123 3122 3122 3122 3122
3122 3122 3120 3121 3121 3121 3120 3123 3121 3120 3122 3121 3121 3121 3121
3120 3120 3122 3122 3121 3121 3121 3121 3121 3121 3122 3121 3122 3122 3122
3120 3120 3121 3122 3121 3120 3121 3122 3121 3121 3122 3121 3121 3121 3121
3121 3121 3121 3121 3121 3121 3121 3121 3121 3122 3121 3122 3122 3122 3122
3121 3121 3120 3122 3121 3121 3121 3120 3121 3121 3122 3121 3121 3121 3121
3120 3120 3120 3122 3121 3121 3121 3121 3121 3121 3122 3121 3125 3125 3125
3120 3120 3121 3122 3121 3121 3121 3121 3121 3122 3121 3122 3122 3122 3122
3121 3121 3120 3122 3121 3121 3121 3121 3121 3122 3122 3121 3122 3122 3122
3121 3121 3122 3122 3121 3121 3120 3121 3121 3122 3122 3122 3122 3122 3122
3120 3120 3120 3122 3121 3120 3121 3121 3121 3122 3122 3121 3122 3121 3121
3121 3121 3120 3122 3121 3120 3121 3120 3121 3122 3122 3121 3121 3121 3121
3121 3121 3121 3122 3121 3120 3121 3121 3121 3122 3122 3121 3121 3121 3121
3122 3122 3121 3122 3121 3120 3120 3121 3121 3122 3122 3121 3122 3122 3122
3120 3120 3120 3122 3122 3120 3120 3121 3123 3121 3122 3121 3121 3121 3121
3120 3120 3120 3122 3120 3121 3121 3120 3121 3121 3122 3121 3121 3121 3121
3120 3120 3120 3122 3121 3121 3121 3120 3121 3122 3121 3121 3121 3121 3121
3120 3120 3120 3122 3121 3120 3121 3121 3121 3122 3121 3121 3121 3121 3121
3121 3121 3122 3122 3120 3120 3121 3120 3122 3121 3122 3121 3122 3122 3122
3122 3122 3120 3120 3121 3120 3121 3121 3121 3122 3122 3122 3121 3121 3121
3121 3121 3120 3122 3121 3120 3121 3121 3121 3122 3122 3121 3121 3121 3121
3121 3121 3120 3122 3120 3120 3121 3120 3121 3122 3122 3122 3122 3122 3122
3121 3121 3120 3122 3122 3121 3120 3120 3121 3122 3121 3122 3121 3121 3121

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 3121 bytes 100%
1,000 3120 bytes -1 byte 100%
10,000 3120 bytes 100%
100,000
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
3131 bytes +11 bytes (+0.35%)
3131 bytes +11 bytes (+0.35%)
3153 bytes +33 bytes (+1.06%) +22 bytes
3147 bytes +27 bytes (+0.87%) +16 bytes
3176 bytes +56 bytes (+1.79%) +45 bytes
3208 bytes +88 bytes (+2.82%) +77 bytes
3235 bytes +115 bytes (+3.69%) +104 bytes
3269 bytes +149 bytes (+4.78%) +138 bytes
3306 bytes +186 bytes (+5.96%) +175 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 2825 bytes -295 bytes (-9.46%)
Brotli (Wikipedia) brotli brotli -q 11 2887 bytes -233 bytes (-7.47%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 3131 bytes +11 bytes (+0.35%)
ZSTD (Wikipedia) zstd zstd -19 3142 bytes +22 bytes (+0.71%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 3199 bytes +79 bytes (+2.53%)
LZMA2 (Wikipedia) xz xz -9 3224 bytes +104 bytes (+3.33%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 3709 bytes +589 bytes (+18.88%)

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