Choose a version:
34% The original file has 37390 bytes (36.5k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 12821 bytes (12.5k, 34%).

After GZIP compression these minified files vary in size:
Boot
  4904 bytes (4.8k)
CDN, direct link
Baidu
  4418 bytes (4.3k)
CDN, direct link
gzip -6 (default)
  4410 bytes (4.3k)
local copy
gzip -9
  4401 bytes (4.3k)
local copy
cdnjs
  4396 bytes (4.3k)
CDN, direct link
unpkg
  4393 bytes (4.3k)
CDN, direct link
cdnhttps
  4386 bytes (4.3k)
CDN, direct link
libdeflate -12
  4282 bytes (4.2k)
local copy
7zip -mx=9 -tgzip
  4280 bytes (4.2k)
local copy
kzip -s0 -rn -b0
  4277 bytes (4.2k)
local copy
pigz -11 -n
  4266 bytes (4.2k)
local copy
Zopfli
  4260 bytes (4.2k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.3.3.min.js

You will automatically get the smallest UnderscoreJS 1.3.3 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 126 bytes by using my UnderscoreJS 1.3.3 Zopfli version instead of the best available CDN (2.96% smaller than cdnhttps, 4260 vs. 4386 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 --mls1024 --bsr8 --lazy --ohh

(found September 8, 2015)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 1024  --mls1024
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/1.3.3/underscore-min.js --location | md5sum
0899f764c85bbcc13206e3dd8318e678  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.3.3.min.zopfli.js.gz | md5sum
0899f764c85bbcc13206e3dd8318e678  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.3.3/underscore-min.js --location | sha1sum
baa693e82387a328abccf9694c6d2db571ac8aca  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.3.3.min.zopfli.js.gz | sha1sum
baa693e82387a328abccf9694c6d2db571ac8aca  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 4904 bytes 0899f764c85bbcc13206e3dd8318e678 (invalid)
cdnjs 4396 bytes 0899f764c85bbcc13206e3dd8318e678 (invalid)
unpkg 4393 bytes 0899f764c85bbcc13206e3dd8318e678 July 11, 2016 @ 16:49
cdnhttps 4386 bytes 0899f764c85bbcc13206e3dd8318e678 December 24, 2015 @ 07:36

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 4418 bytes 13223be203b9c732fbb5766b6ac55dd1 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:

1.8.3, 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
4260 bytes -1 byte zopfli --i10000 --mls1024 --bsr8 --lazy --ohh September 8, 2015 @ 15:07
4261 bytes -1 byte zopfli --i10000 --mls512 --bsr12 --lazy --ohh September 8, 2015 @ 15:01
4262 bytes -1 byte zopfli --i1000 --mls512 --bsr10 --lazy --ohh September 8, 2015 @ 10:53
4263 bytes -1 byte zopfli --i1000 --mls512 --bsr14 --lazy --ohh September 8, 2015 @ 10:53
4264 bytes zopfli --i100 --mls512 --bsr16 --lazy --ohh September 8, 2015 @ 10:32

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
4269 4269 4277 4268 4267 4277 4277 4265 4269 4268 4268 4267 4269 4268 4268
4263 4263 4263 4263 4261 4261 4264 4267 4264 4264 4262 4263 4261 4265 4265
4263 4264 4264 4263 4262 4263 4264 4262 4263 4261 4267 4262 4264 4261 4261
4275 4275 4275 4275 4261 4263 4264 4261 4261 4264 4261 4262 4261 4261 4261
4263 4265 4261 4264 4261 4264 4264 4267 4262 4260 4261 4262 4261 4260 4260
4263 4264 4264 4263 4261 4261 4264 4263 4264 4261 4261 4262 4260 4260 4260
4263 4264 4264 4263 4263 4263 4260 4264 4261 4263 4261 4264 4261 4261 4261
4276 4276 4275 4277 4264 4275 4264 4261 4261 4263 4261 4262 4261 4261 4261
4265 4264 4264 4263 4263 4264 4264 4262 4261 4264 4261 4262 4262 4261 4261
4264 4264 4262 4263 4264 4263 4264 4267 4261 4264 4262 4262 4260 4261 4261
4264 4264 4261 4262 4263 4263 4264 4261 4261 4264 4261 4262 4260 4261 4261
4264 4264 4264 4263 4262 4263 4264 4262 4262 4264 4261 4262 4260 4261 4261
4263 4265 4264 4263 4261 4263 4264 4264 4261 4264 4262 4262 4260 4260 4260
4263 4264 4264 4263 4264 4263 4264 4262 4261 4264 4261 4262 4260 4260 4260
4264 4264 4264 4263 4260 4262 4264 4260 4261 4264 4261 4264 4261 4261 4261
4264 4264 4264 4264 4263 4263 4264 4261 4263 4261 4261 4262 4261 4261 4261
4264 4264 4261 4263 4264 4264 4260 4263 4261 4264 4261 4262 4261 4261 4261
4276 4276 4265 4263 4261 4263 4264 4263 4263 4264 4261 4262 4261 4261 4261
4263 4264 4261 4263 4263 4263 4264 4263 4261 4264 4261 4262 4261 4261 4261
4264 4264 4261 4263 4263 4263 4264 4263 4261 4264 4261 4264 4262 4262 4262
4264 4265 4264 4263 4262 4263 4264 4262 4261 4264 4262 4262 4262 4262 4262
4265 4265 4264 4264 4262 4264 4264 4264 4261 4261 4261 4262 4261 4262 4262
4265 4265 4264 4262 4264 4264 4264 4261 4261 4264 4266 4267 4260 4260 4260

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 4264 bytes 100%
1,000 4262 bytes -2 bytes 100%
10,000 4260 bytes -2 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
4277 bytes +17 bytes (+0.40%)
4278 bytes +18 bytes (+0.42%) +1 byte
4291 bytes +31 bytes (+0.73%) +14 bytes
4290 bytes +30 bytes (+0.70%) +13 bytes
4324 bytes +64 bytes (+1.50%) +47 bytes
4345 bytes +85 bytes (+2.00%) +68 bytes
4376 bytes +116 bytes (+2.72%) +99 bytes
4400 bytes +140 bytes (+3.29%) +123 bytes
4425 bytes +165 bytes (+3.87%) +148 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 3809 bytes -451 bytes (-10.59%)
Brotli (Wikipedia) brotli brotli -q 11 3937 bytes -323 bytes (-7.58%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 4240 bytes -20 bytes (-0.47%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 4281 bytes +21 bytes (+0.49%)
ZSTD (Wikipedia) zstd zstd -19 4304 bytes +44 bytes (+1.03%)
LZMA2 (Wikipedia) xz xz -9 4352 bytes +92 bytes (+2.16%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 4640 bytes +380 bytes (+8.92%)

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