Choose a version:
33% The original file has 43568 bytes (42.5k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 14358 bytes (14.0k, 33%).

After GZIP compression these minified files vary in size:
Boot
  5580 bytes (5.4k)
CDN
jsdelivr
  5580 bytes (5.4k)
CDN
gzip -6 (default)
  4993 bytes (4.9k)
local copy
Baidu
  4989 bytes (4.9k)
CDN
cdnjs
  4984 bytes (4.9k)
CDN
gzip -9
  4982 bytes (4.9k)
local copy
unpkg
  4979 bytes (4.9k)
CDN
cdnhttps
  4969 bytes (4.9k)
CDN
Yandex
  4960 bytes (4.8k)
CDN
7zip -mx=9 -tgzip
  4864 bytes (4.8k)
local copy
libdeflate -12
  4859 bytes (4.7k)
local copy
kzip -s0 -rn -b0
  4856 bytes (4.7k)
local copy
pigz -11 -n
  4847 bytes (4.7k)
local copy
Zopfli
  4842 bytes (4.7k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.5.2.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 1.5.2 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 118 bytes by using my UnderscoreJS 1.5.2 Zopfli version instead of the best available CDN (2.44% smaller than Yandex, 4842 vs. 4960 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 --mls512 --bsr12 --lazy --ohh

(found September 9, 2015)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 512  --mls512
block splitting recursion 12  --bsr12
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/1.5.2/underscore-min.js --location | md5sum
ca26dc8cdf5d413cd8d3b62490e28210  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.5.2.min.zopfli.js.gz | md5sum
ca26dc8cdf5d413cd8d3b62490e28210  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.5.2/underscore-min.js --location | sha1sum
005cbdf96df45798aec80ada6afd7a6f58652181  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.5.2.min.zopfli.js.gz | sha1sum
005cbdf96df45798aec80ada6afd7a6f58652181  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 5580 bytes ca26dc8cdf5d413cd8d3b62490e28210 (invalid)
jsdelivr 5580 bytes ca26dc8cdf5d413cd8d3b62490e28210 (invalid)
cdnjs 4984 bytes ca26dc8cdf5d413cd8d3b62490e28210 (invalid)
unpkg 4979 bytes ca26dc8cdf5d413cd8d3b62490e28210 July 11, 2016 @ 16:49
cdnhttps 4969 bytes ca26dc8cdf5d413cd8d3b62490e28210 December 24, 2015 @ 07:36
Yandex 4960 bytes ca26dc8cdf5d413cd8d3b62490e28210 (invalid)

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 4989 bytes f1b6a32d917945b051781a58a6975af2 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:

1.9.1, 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
4842 bytes -1 byte zopfli --i100000 --mls512 --bsr12 --lazy --ohh September 9, 2015 @ 07:27
4843 bytes zopfli --i100 --mls512 --bsr9 --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 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
4856 4856 4843 4844 4844 4843 4843 4843 4843 4842 4843 4855 4855 4843 4843
4843 4843 4843 4843 4843 4843 4842 4848 4842 4843 4843 4844 4844 4845 4845
4844 4844 4843 4844 4844 4843 4843 4844 4843 4843 4843 4843 4843 4843 4843
4845 4845 4843 4843 4843 4843 4843 4846 4843 4843 4843 4844 4846 4846 4846
4843 4843 4844 4844 4844 4843 4843 4844 4843 4842 4843 4843 4842 4844 4844
4844 4844 4843 4844 4844 4843 4844 4844 4842 4843 4842 4843 4843 4845 4845
4844 4844 4843 4844 4843 4843 4843 4843 4842 4843 4843 4843 4842 4844 4844
4844 4844 4843 4843 4843 4843 4843 4844 4842 4842 4843 4842 4843 4844 4844
4844 4844 4843 4843 4843 4843 4843 4844 4842 4844 4842 4844 4844 4844 4844
4844 4844 4844 4844 4844 4843 4844 4844 4844 4842 4842 4843 4843 4844 4844
4843 4843 4843 4843 4844 4843 4842 4843 4842 4842 4843 4843 4843 4844 4844
4844 4844 4843 4843 4843 4843 4843 4844 4844 4843 4844 4844 4846 4844 4844
4844 4844 4843 4843 4844 4844 4843 4844 4842 4842 4844 4843 4842 4844 4844
4844 4844 4843 4844 4843 4844 4844 4843 4843 4843 4844 4844 4843 4843 4843
4844 4844 4843 4844 4847 4847 4843 4843 4843 4843 4844 4843 4844 4844 4844
4844 4844 4843 4843 4844 4843 4843 4846 4843 4844 4843 4844 4843 4843 4843
4844 4844 4844 4843 4843 4843 4844 4843 4843 4844 4844 4845 4844 4844 4844
4844 4844 4843 4843 4844 4845 4844 4844 4843 4843 4844 4845 4843 4844 4844
4844 4844 4843 4843 4844 4843 4843 4843 4843 4843 4843 4845 4844 4844 4844
4844 4844 4843 4844 4844 4847 4847 4846 4843 4845 4844 4844 4843 4843 4843
4845 4845 4844 4843 4844 4843 4842 4843 4844 4843 4844 4844 4844 4844 4844
4844 4844 4843 4844 4844 4844 4843 4843 4844 4843 4844 4844 4844 4843 4843
4844 4844 4844 4843 4844 4843 4843 4844 4844 4843 4844 4844 4844 4843 4843

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 4843 bytes 100%
1,000 4843 bytes 100%
10,000 4842 bytes -1 byte 100%
100,000 4842 bytes 15.65%
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
4856 bytes +14 bytes (+0.29%)
4856 bytes +14 bytes (+0.29%)
4875 bytes +33 bytes (+0.68%) +19 bytes
4872 bytes +30 bytes (+0.62%) +16 bytes
4903 bytes +61 bytes (+1.26%) +47 bytes
4941 bytes +99 bytes (+2.04%) +85 bytes
4974 bytes +132 bytes (+2.73%) +118 bytes
5015 bytes +173 bytes (+3.57%) +159 bytes
5046 bytes +204 bytes (+4.21%) +190 bytes

Non-DEFLATE Algorithms

Archivers based on completely different compression algorithms often produce superior results.
Unfortunately, browsers only support gzip compression at the moment.
However, support for Brotli is constantly growing - but your browser doesn't support it.
Algorithm Program Parameters Size Compared To Best Zopfli
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 4304 bytes -538 bytes (-11.11%)
Brotli (Wikipedia) brotli brotli -q 11 4514 bytes -328 bytes (-6.77%)
Zstandard (Wikipedia) zstd zstd -19 4855 bytes +13 bytes (+0.27%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 4861 bytes +19 bytes (+0.39%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 4871 bytes +29 bytes (+0.60%)
LZMA2 (Wikipedia) xz xz -9 4920 bytes +78 bytes (+1.61%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 5147 bytes +305 bytes (+6.30%)

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