Choose a version:
35% The original file has 24635 bytes (24.1k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 8542 bytes (8.3k, 35%).

After GZIP compression these minified files vary in size:
Boot
  3226 bytes (3.2k)
CDN
gzip -6 (default)
  2940 bytes (2.9k)
local copy
Baidu
  2933 bytes (2.9k)
CDN
gzip -9
  2932 bytes (2.9k)
local copy
unpkg
  2920 bytes (2.9k)
CDN
cdnjs
  2919 bytes (2.9k)
CDN
cdnhttps
  2916 bytes (2.8k)
CDN
libdeflate -12
  2840 bytes (2.8k)
local copy
7zip -mx=9 -tgzip
  2838 bytes (2.8k)
local copy
kzip -s0 -rn -b1
  2838 bytes (2.8k)
local copy
pigz -11 -n
  2836 bytes (2.8k)
local copy
Zopfli
  2835 bytes (2.8k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.0.3.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 1.0.3 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 81 bytes by using my UnderscoreJS 1.0.3 Zopfli version instead of the best available CDN (2.86% smaller than cdnhttps, 2835 vs. 2916 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 --i100 --mb8 --mls16 --bsr21 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 100  --i100
maximum blocks 8  --mb8
maximum length score 16  --mls16
block splitting recursion 21  --bsr21
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/1.0.3/underscore-min.js --location | md5sum
212c425ad364a0d4e9f195c2ee88d12a  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.0.3.min.zopfli.js.gz | md5sum
212c425ad364a0d4e9f195c2ee88d12a  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.0.3/underscore-min.js --location | sha1sum
97f93241957893ebd7febc5b5bac9a7bd1d94a0f  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.0.3.min.zopfli.js.gz | sha1sum
97f93241957893ebd7febc5b5bac9a7bd1d94a0f  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 3226 bytes 212c425ad364a0d4e9f195c2ee88d12a (invalid)
unpkg 2920 bytes 212c425ad364a0d4e9f195c2ee88d12a July 11, 2016 @ 16:50
cdnjs 2919 bytes 212c425ad364a0d4e9f195c2ee88d12a (invalid)
cdnhttps 2916 bytes 212c425ad364a0d4e9f195c2ee88d12a (invalid)

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 2933 bytes 53c66bf4859af74420042c2dc6873b3a 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
2835 bytes zopfli --i100 --mls16 --bsr21 --lazy --ohh February 2, 2016 @ 19:07

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 @ 16:50.

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
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835
2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835 2835

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 2835 bytes 100%
1,000 2835 bytes 100%
10,000 2835 bytes 100%
100,000 2835 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
2838 bytes +3 bytes (+0.11%)
2838 bytes +3 bytes (+0.11%)
2862 bytes +27 bytes (+0.95%) +24 bytes
2895 bytes +60 bytes (+2.12%) +57 bytes
2905 bytes +70 bytes (+2.47%) +67 bytes
2935 bytes +100 bytes (+3.53%) +97 bytes
2966 bytes +131 bytes (+4.62%) +128 bytes
3005 bytes +170 bytes (+6.00%) +167 bytes
3040 bytes +205 bytes (+7.23%) +202 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 2552 bytes -283 bytes (-9.98%)
Brotli (Wikipedia) brotli brotli -q 11 2633 bytes -202 bytes (-7.13%)
Zstandard (Wikipedia) zstd zstd -19 2853 bytes +18 bytes (+0.63%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 2859 bytes +24 bytes (+0.85%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 2891 bytes +56 bytes (+1.98%)
LZMA2 (Wikipedia) xz xz -9 2924 bytes +89 bytes (+3.14%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 3428 bytes +593 bytes (+20.92%)

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