Choose a version:
33% The original file has 40589 bytes (39.6k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 13430 bytes (13.1k, 33%).

After GZIP compression these minified files vary in size:
Boot
  5154 bytes (5.0k)
CDN, direct link
gzip -6 (default)
  4654 bytes (4.5k)
local copy
Baidu
  4650 bytes (4.5k)
CDN, direct link
cdnjs
  4648 bytes (4.5k)
CDN, direct link
gzip -9
  4638 bytes (4.5k)
local copy
cdnhttps
  4630 bytes (4.5k)
CDN, direct link
unpkg
  4626 bytes (4.5k)
CDN, direct link
7zip -mx=9 -tgzip
  4517 bytes (4.4k)
local copy
libdeflate -12
  4513 bytes (4.4k)
local copy
kzip -s0 -rn -b0
  4509 bytes (4.4k)
local copy
pigz -11 -n
  4504 bytes (4.4k)
local copy
Zopfli
  4499 bytes (4.4k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.4.1.min.js

You will automatically get the smallest UnderscoreJS 1.4.1 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 127 bytes by using my UnderscoreJS 1.4.1 Zopfli version instead of the best available CDN (2.82% smaller than unpkg, 4499 vs. 4626 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 --mls256 --bsr13 --lazy --ohh

(found September 20, 2015)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 256  --mls256
block splitting recursion 13  --bsr13
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/1.4.1/underscore-min.js --location | md5sum
a8167cbe4acedc5f0132f7b087c24bd0  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.4.1.min.zopfli.js.gz | md5sum
a8167cbe4acedc5f0132f7b087c24bd0  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.4.1/underscore-min.js --location | sha1sum
42394e7c9142b938e1d807c643d24107d7d9d317  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.4.1.min.zopfli.js.gz | sha1sum
42394e7c9142b938e1d807c643d24107d7d9d317  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 5154 bytes a8167cbe4acedc5f0132f7b087c24bd0 (invalid)
cdnjs 4648 bytes a8167cbe4acedc5f0132f7b087c24bd0 (invalid)
cdnhttps 4630 bytes a8167cbe4acedc5f0132f7b087c24bd0 December 24, 2015 @ 07:36
unpkg 4626 bytes a8167cbe4acedc5f0132f7b087c24bd0 July 11, 2016 @ 16:49

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 4650 bytes 31e38eceaa491ef56199e268faab5e12 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
4499 bytes -1 byte zopfli --i10000 --mls256 --bsr13 --lazy --ohh September 20, 2015 @ 16:48
4500 bytes -1 byte zopfli --i10000 --mls512 --bsr4 --lazy --ohh September 8, 2015 @ 14:24
4501 bytes zopfli --i100 --mls4096 --bsr6 --lazy --ohh September 8, 2015 @ 10:33

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 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
4508 4508 4500 4501 4501 4500 4507 4501 4500 4500 4500 4501 4501 4502 4502
4507 4507 4507 4506 4506 4506 4507 4500 4500 4501 4509 4501 4503 4500 4500
4504 4504 4504 4504 4504 4502 4507 4500 4500 4500 4501 4500 4501 4500 4500
4504 4504 4505 4505 4504 4504 4505 4507 4500 4508 4501 4503 4501 4501 4501
4509 4509 4501 4500 4502 4502 4507 4499 4500 4501 4501 4502 4503 4500 4500
4505 4505 4505 4502 4502 4500 4504 4500 4500 4500 4500 4503 4501 4504 4504
4505 4505 4500 4500 4500 4500 4505 4500 4501 4500 4501 4502 4502 4501 4501
4505 4505 4500 4502 4503 4511 4505 4500 4500 4500 4501 4502 4501 4500 4500
4510 4510 4500 4502 4502 4502 4502 4500 4500 4500 4501 4504 4500 4501 4501
4507 4507 4500 4502 4502 4502 4503 4499 4500 4500 4501 4500 4501 4501 4501
4502 4502 4503 4500 4500 4501 4503 4500 4501 4503 4501 4504 4501 4500 4500
4504 4504 4502 4500 4500 4500 4504 4500 4500 4500 4501 4502 4500 4500 4500
4504 4504 4504 4500 4500 4511 4504 4500 4500 4500 4501 4507 4503 4502 4502
4504 4504 4500 4502 4502 4503 4500 4499 4500 4500 4505 4504 4503 4501 4501
4504 4504 4502 4502 4503 4502 4500 4500 4500 4504 4504 4502 4501 4501 4501
4505 4505 4504 4500 4502 4500 4500 4501 4500 4500 4501 4500 4501 4501 4501
4504 4504 4502 4503 4504 4500 4504 4500 4500 4500 4501 4502 4500 4501 4501
4505 4505 4502 4500 4500 4500 4500 4500 4500 4500 4501 4502 4502 4502 4502
4502 4502 4502 4502 4500 4502 4500 4500 4500 4500 4504 4500 4501 4502 4502
4502 4502 4502 4502 4503 4502 4504 4504 4502 4500 4504 4501 4500 4501 4501
4505 4505 4500 4500 4501 4500 4500 4500 4500 4501 4501 4500 4501 4501 4501
4504 4504 4502 4502 4501 4501 4501 4501 4500 4501 4503 4500 4502 4502 4502
4505 4505 4506 4500 4500 4500 4500 4500 4505 4500 4501 4500 4500 4500 4500

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 4501 bytes 100%
1,000 4500 bytes -1 byte 100%
10,000 4499 bytes -1 byte 100%
100,000 4499 bytes 0.87%
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
4509 bytes +10 bytes (+0.22%)
4509 bytes +10 bytes (+0.22%)
4527 bytes +28 bytes (+0.62%) +18 bytes
4529 bytes +30 bytes (+0.67%) +20 bytes
4561 bytes +62 bytes (+1.38%) +52 bytes
4597 bytes +98 bytes (+2.18%) +88 bytes
4633 bytes +134 bytes (+2.98%) +124 bytes
4660 bytes +161 bytes (+3.58%) +151 bytes
4691 bytes +192 bytes (+4.27%) +182 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 4015 bytes -484 bytes (-10.76%)
Brotli (Wikipedia) brotli brotli -q 11 4180 bytes -319 bytes (-7.09%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 4418 bytes -81 bytes (-1.80%)
ZSTD (Wikipedia) zstd zstd -19 4521 bytes +22 bytes (+0.49%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 4536 bytes +37 bytes (+0.82%)
LZMA2 (Wikipedia) xz xz -9 4588 bytes +89 bytes (+1.98%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 4864 bytes +365 bytes (+8.11%)

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