Choose a version:
33% The original file has 47559 bytes (46.4k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 15626 bytes (15.3k, 33%).

After GZIP compression these minified files vary in size:
Boot
  5986 bytes (5.8k)
CDN, direct link
jsdelivr
  5986 bytes (5.8k)
CDN, direct link
Baidu
  5335 bytes (5.2k)
CDN, direct link
cdnjs
  5328 bytes (5.2k)
CDN, direct link
gzip -6 (default)
  5324 bytes (5.2k)
local copy
gzip -9
  5313 bytes (5.2k)
local copy
cdnhttps
  5300 bytes (5.2k)
CDN, direct link
Yandex
  5293 bytes (5.2k)
CDN, direct link
unpkg
  5289 bytes (5.2k)
CDN, direct link
7zip -mx=9 -tgzip
  5160 bytes (5.0k)
local copy
libdeflate -12
  5158 bytes (5.0k)
local copy
kzip -s0 -rn -b1
  5151 bytes (5.0k)
local copy
pigz -11 -n
  5144 bytes (5.0k)
local copy
Zopfli
  5138 bytes (5.0k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.7.0.min.js

You will automatically get the smallest UnderscoreJS 1.7.0 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 151 bytes by using my UnderscoreJS 1.7.0 Zopfli version instead of the best available CDN (2.94% smaller than unpkg, 5138 vs. 5289 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 --i100000 --mb8 --mls8192 --bsr16 --lazy --ohh

(found September 9, 2015)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 8192  --mls8192
block splitting recursion 16  --bsr16
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/1.7.0/underscore-min.js --location | md5sum
137af05d496f59d468d1ffbce32f375d  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.7.0.min.zopfli.js.gz | md5sum
137af05d496f59d468d1ffbce32f375d  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.7.0/underscore-min.js --location | sha1sum
fb26909af4ad2a6c240b9aa4b35bb983cf4b20e4  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.7.0.min.zopfli.js.gz | sha1sum
fb26909af4ad2a6c240b9aa4b35bb983cf4b20e4  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 5986 bytes 137af05d496f59d468d1ffbce32f375d (invalid)
jsdelivr 5986 bytes 137af05d496f59d468d1ffbce32f375d (invalid)
cdnjs 5328 bytes 137af05d496f59d468d1ffbce32f375d (invalid)
cdnhttps 5300 bytes 137af05d496f59d468d1ffbce32f375d December 24, 2015 @ 07:36
Yandex 5293 bytes 137af05d496f59d468d1ffbce32f375d December 22, 2015 @ 17:09
unpkg 5289 bytes 137af05d496f59d468d1ffbce32f375d July 11, 2016 @ 16:49

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 5335 bytes 963bd48ea80c28f4481f86c03256fa0b 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
5138 bytes -1 byte zopfli --i100000 --mls8192 --bsr16 --lazy --ohh September 9, 2015 @ 03:51
5139 bytes -1 byte zopfli --i1000 --mls4096 --bsr16 --lazy --ohh September 8, 2015 @ 10:40
5140 bytes zopfli --i100 --mls2048 --bsr10 --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 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
5139 5139 5141 5141 5140 5140 5139 5140 5142 5139 5139 5140 5141 5143 5143
5151 5151 5141 5139 5139 5142 5143 5143 5143 5142 5140 5140 5140 5139 5139
5140 5140 5141 5140 5139 5139 5139 5140 5140 5140 5140 5141 5140 5143 5143
5144 5144 5141 5140 5140 5141 5139 5141 5141 5139 5141 5141 5141 5142 5142
5140 5140 5141 5141 5140 5139 5139 5140 5140 5139 5140 5141 5140 5139 5139
5139 5139 5141 5141 5141 5139 5139 5139 5139 5140 5140 5140 5140 5139 5139
5139 5139 5139 5140 5140 5139 5148 5140 5139 5140 5139 5140 5140 5139 5139
5138 5138 5139 5140 5140 5139 5139 5140 5139 5140 5140 5140 5140 5143 5143
5144 5144 5141 5140 5139 5139 5140 5140 5139 5140 5140 5140 5140 5144 5144
5151 5151 5141 5140 5140 5139 5139 5140 5140 5140 5141 5140 5140 5143 5143
5145 5145 5139 5141 5141 5139 5139 5140 5139 5140 5140 5140 5139 5141 5141
5143 5143 5139 5140 5140 5139 5139 5139 5140 5139 5141 5142 5142 5142 5142
5148 5148 5149 5141 5140 5139 5139 5140 5139 5140 5140 5139 5138 5143 5143
5151 5151 5140 5140 5139 5139 5139 5140 5140 5140 5141 5140 5141 5143 5143
5151 5151 5139 5140 5139 5139 5139 5140 5140 5140 5140 5140 5142 5139 5139
5140 5140 5139 5140 5140 5139 5139 5140 5140 5141 5141 5141 5141 5140 5140
5151 5151 5141 5140 5139 5139 5139 5140 5140 5141 5141 5141 5141 5143 5143
5147 5147 5139 5140 5140 5139 5139 5140 5140 5141 5141 5141 5141 5143 5143
5138 5138 5139 5142 5139 5139 5141 5140 5140 5141 5141 5141 5141 5141 5141
5138 5138 5139 5141 5140 5141 5141 5140 5140 5140 5141 5141 5141 5141 5141
5145 5145 5141 5141 5139 5141 5139 5140 5140 5140 5142 5140 5142 5143 5143
5145 5145 5141 5141 5140 5141 5139 5141 5141 5141 5141 5139 5141 5140 5140
5139 5139 5140 5140 5140 5139 5139 5140 5140 5141 5140 5140 5140 5140 5140

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 5139 bytes 100%
1,000 5139 bytes 100%
10,000 5138 bytes -1 byte 100%
100,000 5138 bytes 17.68%
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
5151 bytes +13 bytes (+0.25%)
5151 bytes +13 bytes (+0.25%)
5174 bytes +36 bytes (+0.70%) +23 bytes
5175 bytes +37 bytes (+0.72%) +24 bytes
5201 bytes +63 bytes (+1.23%) +50 bytes
5232 bytes +94 bytes (+1.83%) +81 bytes
5267 bytes +129 bytes (+2.51%) +116 bytes
5303 bytes +165 bytes (+3.21%) +152 bytes
5342 bytes +204 bytes (+3.97%) +191 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 4554 bytes -584 bytes (-11.37%)
Brotli (Wikipedia) brotli brotli -q 11 4795 bytes -343 bytes (-6.68%)
ZSTD (Wikipedia) zstd zstd -19 5147 bytes +9 bytes (+0.18%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 5167 bytes +29 bytes (+0.56%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 5193 bytes +55 bytes (+1.07%)
LZMA2 (Wikipedia) xz xz -9 5200 bytes +62 bytes (+1.21%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 5366 bytes +228 bytes (+4.44%)

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