Choose a version:
35% The original file has 34498 bytes (33.7k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 11911 bytes (11.6k, 35%).

After GZIP compression these minified files vary in size:
Boot
  4531 bytes (4.4k)
CDN
Baidu
  4067 bytes (4.0k)
CDN
gzip -6 (default)
  4052 bytes (4.0k)
local copy
gzip -9
  4046 bytes (4.0k)
local copy
cdnjs
  4045 bytes (4.0k)
CDN
unpkg
  4032 bytes (3.9k)
CDN
cdnhttps
  4028 bytes (3.9k)
CDN
libdeflate -12
  3945 bytes (3.9k)
local copy
7zip -mx=9 -tgzip
  3938 bytes (3.8k)
local copy
kzip -s0 -rn -b0
  3938 bytes (3.8k)
local copy
pigz -11 -n
  3931 bytes (3.8k)
local copy
Zopfli
  3925 bytes (3.8k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.2.2.min.js

You will automatically get the smallest UnderscoreJS 1.2.2 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 103 bytes by using my UnderscoreJS 1.2.2 Zopfli version instead of the best available CDN (2.62% smaller than cdnhttps, 3925 vs. 4028 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 --bsr9 --lazy --ohh

(found September 8, 2015)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 1024  --mls1024
block splitting recursion 9  --bsr9
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/1.2.2/underscore-min.js --location | md5sum
f0be4eb0429e119c9d369e6ec2ac5f08  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.2.2.min.zopfli.js.gz | md5sum
f0be4eb0429e119c9d369e6ec2ac5f08  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.2.2/underscore-min.js --location | sha1sum
1185da1043863517c6aa0a4e0a3bc433dfb98313  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.2.2.min.zopfli.js.gz | sha1sum
1185da1043863517c6aa0a4e0a3bc433dfb98313  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 4531 bytes f0be4eb0429e119c9d369e6ec2ac5f08 (invalid)
cdnjs 4045 bytes f0be4eb0429e119c9d369e6ec2ac5f08 (invalid)
unpkg 4032 bytes f0be4eb0429e119c9d369e6ec2ac5f08 July 11, 2016 @ 16:49
cdnhttps 4028 bytes f0be4eb0429e119c9d369e6ec2ac5f08 December 24, 2015 @ 07:36

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 4067 bytes aa20654a415040ea0a8146dc0a048aac 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
3925 bytes -1 byte zopfli --i10000 --mls1024 --bsr9 --lazy --ohh September 8, 2015 @ 16:03
3926 bytes -2 bytes zopfli --i1000 --mls512 --bsr9 --lazy --ohh September 8, 2015 @ 10:58
3928 bytes zopfli --i100 --mls8192 --bsr12 --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.
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
3930 3930 3931 3930 3930 3930 3932 3942 3942 3929 3930 3931 3930 3929 3929
3928 3928 3930 3929 3932 3933 3932 3933 3933 3926 3932 3932 3931 3932 3932
3928 3928 3930 3928 3929 3927 3930 3930 3928 3928 3931 3926 3926 3928 3928
3933 3933 3930 3927 3932 3932 3928 3934 3927 3927 3926 3926 3927 3927 3927
3928 3928 3929 3929 3929 3928 3930 3927 3930 3926 3931 3926 3926 3927 3927
3927 3927 3929 3928 3929 3933 3926 3926 3925 3925 3933 3927 3927 3927 3927
3942 3942 3927 3926 3932 3929 3928 3926 3926 3926 3926 3926 3927 3927 3927
3941 3941 3928 3928 3929 3932 3928 3931 3930 3925 3930 3928 3927 3927 3927
3928 3928 3928 3928 3929 3942 3928 3931 3930 3925 3931 3927 3927 3927 3927
3928 3928 3930 3929 3929 3929 3929 3928 3928 3927 3927 3926 3927 3927 3927
3933 3933 3928 3931 3932 3935 3933 3930 3931 3928 3931 3930 3927 3927 3927
3928 3928 3928 3928 3929 3929 3928 3930 3926 3928 3926 3926 3927 3927 3927
3928 3928 3928 3928 3929 3932 3927 3927 3928 3926 3927 3927 3927 3927 3927
3942 3942 3928 3928 3929 3932 3930 3930 3930 3925 3930 3926 3927 3927 3927
3928 3928 3928 3928 3929 3929 3928 3926 3926 3925 3926 3928 3927 3927 3927
3928 3928 3928 3928 3932 3931 3928 3926 3926 3925 3930 3926 3927 3927 3927
3928 3928 3928 3928 3929 3929 3928 3928 3928 3925 3927 3927 3927 3927 3927
3928 3928 3928 3929 3929 3932 3928 3926 3926 3925 3926 3928 3927 3927 3927
3928 3928 3928 3933 3929 3932 3928 3930 3930 3927 3930 3928 3925 3926 3926
3928 3928 3928 3928 3929 3928 3929 3927 3927 3925 3926 3927 3928 3927 3927
3928 3928 3929 3928 3929 3942 3928 3931 3930 3925 3931 3927 3926 3927 3927
3928 3928 3928 3929 3929 3929 3930 3926 3926 3925 3931 3926 3927 3927 3927
3928 3928 3928 3929 3928 3929 3927 3930 3928 3926 3930 3928 3927 3927 3927

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 3928 bytes 100%
1,000 3926 bytes -2 bytes 100%
10,000 3925 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
3938 bytes +13 bytes (+0.33%)
3938 bytes +13 bytes (+0.33%)
3954 bytes +29 bytes (+0.74%) +16 bytes
3955 bytes +30 bytes (+0.76%) +17 bytes
3987 bytes +62 bytes (+1.58%) +49 bytes
4018 bytes +93 bytes (+2.37%) +80 bytes
4041 bytes +116 bytes (+2.96%) +103 bytes
4066 bytes +141 bytes (+3.59%) +128 bytes
4101 bytes +176 bytes (+4.48%) +163 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 3515 bytes -410 bytes (-10.45%)
Brotli (Wikipedia) brotli brotli -q 11 3617 bytes -308 bytes (-7.85%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 3894 bytes -31 bytes (-0.79%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 3947 bytes +22 bytes (+0.56%)
ZSTD (Wikipedia) zstd zstd -19 3959 bytes +34 bytes (+0.87%)
LZMA2 (Wikipedia) xz xz -9 4016 bytes +91 bytes (+2.32%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 4375 bytes +450 bytes (+11.46%)

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