Choose a version:
34% The original file has 37382 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 12815 bytes (12.5k, 34%).

After GZIP compression these minified files vary in size:
Boot
  4901 bytes (4.8k)
CDN
Baidu
  4415 bytes (4.3k)
CDN
gzip -6 (default)
  4406 bytes (4.3k)
local copy
gzip -9
  4397 bytes (4.3k)
local copy
cdnjs
  4392 bytes (4.3k)
CDN
unpkg
  4391 bytes (4.3k)
CDN
cdnhttps
  4382 bytes (4.3k)
CDN
libdeflate -12
  4279 bytes (4.2k)
local copy
7zip -mx=9 -tgzip
  4277 bytes (4.2k)
local copy
kzip -s0 -rn -b0
  4274 bytes (4.2k)
local copy
pigz -11 -n
  4262 bytes (4.2k)
local copy
Zopfli
  4258 bytes (4.2k)
local copy
Zopfli (defluff)
  4257 bytes (4.2k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.3.2.min.js

You will automatically get the smallest UnderscoreJS 1.3.2 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 124 bytes by using my UnderscoreJS 1.3.2 Zopfli version instead of the best available CDN (2.91% smaller than cdnhttps, 4258 vs. 4382 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 --mls512 --bsr11 --lazy --ohh

(found September 8, 2015)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 512  --mls512
block splitting recursion 11  --bsr11
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

Even Smaller Files Thanks To Defluff

Zopfli's output can be further optimized by the defluff tool.
In this particular case, defluff saves 1 more byte (4257 bytes).

Verify file integrity

After decompression, my uncompressed files are identical to the original ones:

MD5:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.3.2/underscore-min.js --location | md5sum
a36fc61e8437815d62c58c520117a872  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.3.2.min.zopfli.js.gz | md5sum
a36fc61e8437815d62c58c520117a872  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.3.2/underscore-min.js --location | sha1sum
ede1d022640f26eaa6109da9e924670f6ed14cce  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.3.2.min.zopfli.js.gz | sha1sum
ede1d022640f26eaa6109da9e924670f6ed14cce  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 4901 bytes a36fc61e8437815d62c58c520117a872 (invalid)
cdnjs 4392 bytes a36fc61e8437815d62c58c520117a872 (invalid)
unpkg 4391 bytes a36fc61e8437815d62c58c520117a872 July 11, 2016 @ 16:49
cdnhttps 4382 bytes a36fc61e8437815d62c58c520117a872 December 24, 2015 @ 07:36

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 4415 bytes b00b6a74b64a686c2ae9a07dc38565ee 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
4258 bytes -1 byte zopfli --i10000 --mls512 --bsr11 --lazy --ohh September 8, 2015 @ 15:13
4259 bytes -2 bytes zopfli --i1000 --mls512 --bsr11 --lazy --ohh September 8, 2015 @ 10:54
4261 bytes zopfli --i100 --mls2048 --bsr6 --lazy --ohh September 8, 2015 @ 10:34

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
4267 4266 4267 4266 4266 4274 4274 4261 4274 4266 4267 4266 4267 4266 4266
4261 4261 4261 4261 4258 4258 4261 4258 4261 4259 4258 4258 4260 4260 4260
4260 4260 4260 4260 4260 4260 4261 4258 4261 4260 4259 4259 4258 4259 4259
4272 4272 4273 4273 4258 4260 4260 4258 4258 4260 4259 4259 4259 4259 4259
4260 4258 4258 4258 4259 4260 4259 4258 4258 4259 4259 4258 4258 4258 4258
4260 4261 4261 4261 4259 4258 4258 4258 4258 4258 4259 4258 4258 4261 4261
4258 4259 4261 4261 4260 4260 4259 4258 4258 4260 4259 4259 4259 4259 4259
4273 4273 4272 4273 4262 4273 4259 4258 4258 4258 4259 4259 4259 4259 4259
4258 4262 4261 4261 4260 4260 4259 4259 4258 4260 4259 4258 4259 4259 4259
4260 4260 4258 4260 4260 4260 4260 4264 4258 4260 4259 4259 4258 4258 4258
4260 4261 4258 4261 4260 4260 4261 4258 4258 4260 4258 4259 4258 4258 4258
4260 4258 4262 4261 4259 4260 4260 4258 4258 4260 4259 4258 4258 4259 4259
4259 4260 4262 4261 4258 4260 4260 4258 4258 4260 4258 4258 4258 4258 4258
4261 4261 4258 4261 4260 4260 4261 4259 4259 4260 4258 4258 4258 4258 4258
4258 4262 4262 4261 4260 4260 4260 4258 4258 4260 4259 4259 4259 4259 4259
4258 4261 4258 4258 4258 4260 4259 4258 4258 4259 4259 4259 4259 4259 4259
4258 4259 4258 4261 4260 4260 4259 4259 4259 4260 4259 4259 4259 4259 4259
4274 4273 4258 4261 4260 4261 4260 4258 4261 4260 4259 4259 4259 4259 4259
4260 4261 4258 4258 4260 4260 4259 4258 4258 4260 4259 4259 4259 4259 4259
4258 4260 4258 4261 4260 4260 4259 4258 4258 4260 4259 4259 4259 4259 4259
4261 4261 4259 4261 4259 4260 4258 4258 4258 4259 4259 4259 4259 4259 4259
4259 4261 4258 4261 4259 4258 4261 4262 4259 4260 4258 4258 4259 4259 4259
4258 4261 4263 4258 4260 4258 4260 4258 4259 4260 4258 4258 4258 4258 4258

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 4261 bytes 100%
1,000 4259 bytes -2 bytes 100%
10,000 4258 bytes -1 byte 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
4274 bytes +16 bytes (+0.38%)
4274 bytes +16 bytes (+0.38%)
4286 bytes +28 bytes (+0.66%) +12 bytes
4286 bytes +28 bytes (+0.66%) +12 bytes
4319 bytes +61 bytes (+1.43%) +45 bytes
4340 bytes +82 bytes (+1.93%) +66 bytes
4372 bytes +114 bytes (+2.68%) +98 bytes
4393 bytes +135 bytes (+3.17%) +119 bytes
4421 bytes +163 bytes (+3.83%) +147 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 3805 bytes -453 bytes (-10.64%)
Brotli (Wikipedia) brotli brotli -q 11 3931 bytes -327 bytes (-7.68%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 4235 bytes -23 bytes (-0.54%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 4281 bytes +23 bytes (+0.54%)
ZSTD (Wikipedia) zstd zstd -19 4300 bytes +42 bytes (+0.99%)
LZMA2 (Wikipedia) xz xz -9 4348 bytes +90 bytes (+2.11%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 4635 bytes +377 bytes (+8.85%)

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