Choose a version:
35% The original file has 24701 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 8628 bytes (8.4k, 35%).

After GZIP compression these minified files vary in size:
Boot
  3253 bytes (3.2k)
CDN
gzip -6 (default)
  2971 bytes (2.9k)
local copy
Baidu
  2967 bytes (2.9k)
CDN
gzip -9
  2964 bytes (2.9k)
local copy
cdnjs
  2954 bytes (2.9k)
CDN
cdnhttps
  2947 bytes (2.9k)
CDN
libdeflate -12
  2870 bytes (2.8k)
local copy
7zip -mx=9 -tgzip
  2869 bytes (2.8k)
local copy
kzip -s0 -rn -b0
  2867 bytes (2.8k)
local copy
pigz -11 -n
  2866 bytes (2.8k)
local copy
Zopfli
  2864 bytes (2.8k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.0.1.min.js

You will automatically get the smallest UnderscoreJS 1.0.1 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 83 bytes by using my UnderscoreJS 1.0.1 Zopfli version instead of the best available CDN (2.90% smaller than cdnhttps, 2864 vs. 2947 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 --i1000 --mb8 --mls16384 --bsr13 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 1000  --i1000
maximum blocks 8  --mb8
maximum length score 16384  --mls16384
block splitting recursion 13  --bsr13
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/1.0.1/underscore-min.js --location | md5sum
d4052e0252f869c7b55ed7bc0544a2e5  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.0.1.min.zopfli.js.gz | md5sum
d4052e0252f869c7b55ed7bc0544a2e5  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.0.1/underscore-min.js --location | sha1sum
710c3a99f0bd456c2fd6dcbcbd1e500e0646ec19  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.0.1.min.zopfli.js.gz | sha1sum
710c3a99f0bd456c2fd6dcbcbd1e500e0646ec19  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 3253 bytes d4052e0252f869c7b55ed7bc0544a2e5 (invalid)
cdnjs 2954 bytes d4052e0252f869c7b55ed7bc0544a2e5 (invalid)
cdnhttps 2947 bytes d4052e0252f869c7b55ed7bc0544a2e5 (invalid)

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 2967 bytes 5356dab7f73f272b96ea0ced070b74ef 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
2864 bytes -2 bytes zopfli --i1000 --mls16384 --bsr13 --lazy --ohh February 2, 2016 @ 19:45
2866 bytes zopfli --i100 --mls1024 --bsr4 --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 June 16, 2016 @ 14:40.

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
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864
2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864 2864

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 2866 bytes 100%
1,000 2864 bytes -2 bytes 100%
10,000 2864 bytes 100%
100,000 2864 bytes 0.87%
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
2867 bytes +3 bytes (+0.10%)
2867 bytes +3 bytes (+0.10%)
2898 bytes +34 bytes (+1.19%) +31 bytes
2932 bytes +68 bytes (+2.37%) +65 bytes
2946 bytes +82 bytes (+2.86%) +79 bytes
2975 bytes +111 bytes (+3.88%) +108 bytes
3008 bytes +144 bytes (+5.03%) +141 bytes
3044 bytes +180 bytes (+6.28%) +177 bytes
3080 bytes +216 bytes (+7.54%) +213 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 2578 bytes -286 bytes (-9.99%)
Brotli (Wikipedia) brotli brotli -q 11 2664 bytes -200 bytes (-6.98%)
ZSTD (Wikipedia) zstd zstd -19 2882 bytes +18 bytes (+0.63%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 2904 bytes +40 bytes (+1.40%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 2917 bytes +53 bytes (+1.85%)
LZMA2 (Wikipedia) xz xz -9 2956 bytes +92 bytes (+3.21%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 3458 bytes +594 bytes (+20.74%)

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