Choose a version:
37% The original file has 22648 bytes (22.1k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 8335 bytes (8.1k, 37%).

After GZIP compression these minified files vary in size:
Boot
  3034 bytes (3.0k)
CDN
gzip -6 (default)
  2745 bytes (2.7k)
local copy
gzip -9
  2743 bytes (2.7k)
local copy
cdnjs
  2729 bytes (2.7k)
CDN
cdnhttps
  2721 bytes (2.7k)
CDN
libdeflate -12
  2665 bytes (2.6k)
local copy
7zip -mx=9 -tgzip
  2663 bytes (2.6k)
local copy
pigz -11 -n
  2662 bytes (2.6k)
local copy
kzip -s0 -rn -b1
  2661 bytes (2.6k)
local copy
Zopfli
  2657 bytes (2.6k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.6.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 0.5.6 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 64 bytes by using my UnderscoreJS 0.5.6 Zopfli version instead of the best available CDN (2.41% smaller than cdnhttps, 2657 vs. 2721 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 --mls2 --bsr6 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 2  --mls2
block splitting recursion 6  --bsr6
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/0.5.6/underscore-min.js --location | md5sum
e0003fddb33466d973b3ca0c5dda5cbe  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.6.min.zopfli.js.gz | md5sum
e0003fddb33466d973b3ca0c5dda5cbe  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.5.6/underscore-min.js --location | sha1sum
d8032fefab9d2b9fc8aa81834a018fb169c04ac8  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.6.min.zopfli.js.gz | sha1sum
d8032fefab9d2b9fc8aa81834a018fb169c04ac8  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 3034 bytes e0003fddb33466d973b3ca0c5dda5cbe (invalid)
cdnjs 2729 bytes e0003fddb33466d973b3ca0c5dda5cbe (invalid)
cdnhttps 2721 bytes e0003fddb33466d973b3ca0c5dda5cbe (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
2657 bytes -1 byte zopfli --i10000 --mls2 --bsr6 --lazy --ohh February 2, 2016 @ 23:16
2658 bytes zopfli --i100 --mls16 --bsr19 --lazy --ohh February 2, 2016 @ 20:58

If there are multiple parameter sets yielding the same compressed size, only the first one found is shown.

Most recent activity on June 16, 2016 @ 14:36.

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
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658
2657 2657 2658 2658 2658 2658 2657 2658 2658 2657 2657 2658 2658 2658 2658

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 2658 bytes 100%
1,000 2658 bytes 100%
10,000 2657 bytes -1 byte 100%
100,000 2657 bytes 0.29%
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
2661 bytes +4 bytes (+0.15%)
2661 bytes +4 bytes (+0.15%)
2683 bytes +26 bytes (+0.98%) +22 bytes
2718 bytes +61 bytes (+2.30%) +57 bytes
2729 bytes +72 bytes (+2.71%) +68 bytes
2756 bytes +99 bytes (+3.73%) +95 bytes
2787 bytes +130 bytes (+4.89%) +126 bytes
2823 bytes +166 bytes (+6.25%) +162 bytes
2858 bytes +201 bytes (+7.56%) +197 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 2388 bytes -269 bytes (-10.12%)
Brotli (Wikipedia) brotli brotli -q 11 2450 bytes -207 bytes (-7.79%)
Zstandard (Wikipedia) zstd zstd -19 2674 bytes +17 bytes (+0.64%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 2716 bytes +59 bytes (+2.22%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 2735 bytes +78 bytes (+2.94%)
LZMA2 (Wikipedia) xz xz -9 2756 bytes +99 bytes (+3.73%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 3281 bytes +624 bytes (+23.49%)

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