Choose a version:
32% The original file has 41417 bytes (40.4k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 13450 bytes (13.1k, 32%).

After GZIP compression these minified files vary in size:
Boot
  5133 bytes (5.0k)
CDN, direct link
jsdelivr
  5133 bytes (5.0k)
CDN, direct link
gzip -6 (default)
  4613 bytes (4.5k)
local copy
Baidu
  4608 bytes (4.5k)
CDN, direct link
cdnjs
  4608 bytes (4.5k)
CDN, direct link
gzip -9
  4601 bytes (4.5k)
local copy
unpkg
  4591 bytes (4.5k)
CDN, direct link
Sina
  4589 bytes (4.5k)
CDN, direct link
cdnhttps
  4589 bytes (4.5k)
CDN, direct link
7zip -mx=9 -tgzip
  4484 bytes (4.4k)
local copy
libdeflate -12
  4481 bytes (4.4k)
local copy
pigz -11 -n
  4480 bytes (4.4k)
local copy
kzip -s0 -rn -b0
  4477 bytes (4.4k)
local copy
Zopfli
  4477 bytes (4.4k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.4.4.min.js

You will automatically get the smallest UnderscoreJS 1.4.4 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 112 bytes by using my UnderscoreJS 1.4.4 Zopfli version instead of the best available CDN (2.50% smaller than cdnhttps, 4477 vs. 4589 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 --mls4096 --bsr4 --lazy --ohh

(found September 8, 2015)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 4096  --mls4096
block splitting recursion 4  --bsr4
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/1.4.4/underscore-min.js --location | md5sum
6dc59d3a72ad547168cf823c2fddc728  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.4.4.min.zopfli.js.gz | md5sum
6dc59d3a72ad547168cf823c2fddc728  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.4.4/underscore-min.js --location | sha1sum
048c69640ac8f0a3add4f2fd219a2fcb12a27d73  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.4.4.min.zopfli.js.gz | sha1sum
048c69640ac8f0a3add4f2fd219a2fcb12a27d73  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 5133 bytes 6dc59d3a72ad547168cf823c2fddc728 (invalid)
jsdelivr 5133 bytes 6dc59d3a72ad547168cf823c2fddc728 (invalid)
Baidu 4608 bytes 6dc59d3a72ad547168cf823c2fddc728 (invalid)
cdnjs 4608 bytes 6dc59d3a72ad547168cf823c2fddc728 (invalid)
unpkg 4591 bytes 6dc59d3a72ad547168cf823c2fddc728 July 11, 2016 @ 16:49
Sina 4589 bytes 6dc59d3a72ad547168cf823c2fddc728 (invalid)
cdnhttps 4589 bytes 6dc59d3a72ad547168cf823c2fddc728 December 24, 2015 @ 07:36

Note: only the MD5 hashes are shown to keep things simple.

Other Versions

Available UnderscoreJS versions at minime.stephan-brumme.com:

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
4477 bytes -1 byte zopfli --i10000 --mls4096 --bsr4 --lazy --ohh September 8, 2015 @ 13:43
4478 bytes -1 byte zopfli --i1000 --mls512 --bsr2 --lazy --ohh September 8, 2015 @ 10:47
4479 bytes zopfli --i100 --mls4096 --bsr9 --lazy --ohh September 8, 2015 @ 10:32

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
4477 4477 4484 4478 4478 4477 4477 4478 4478 4478 4479 4477 4483 4483 4483
4477 4477 4483 4478 4478 4477 4477 4478 4478 4478 4479 4477 4477 4477 4477
4477 4477 4483 4478 4478 4477 4477 4478 4478 4478 4479 4477 4480 4481 4481
4477 4477 4482 4478 4478 4477 4477 4478 4478 4478 4479 4477 4480 4482 4482
4477 4477 4483 4478 4478 4477 4477 4478 4478 4478 4479 4477 4483 4482 4482
4483 4483 4481 4478 4478 4477 4477 4478 4478 4478 4479 4477 4481 4481 4481
4483 4483 4481 4478 4478 4477 4477 4478 4478 4478 4479 4477 4481 4481 4481
4477 4477 4483 4478 4478 4477 4477 4478 4478 4478 4479 4477 4483 4483 4483
4477 4477 4483 4478 4478 4477 4477 4478 4478 4478 4479 4477 4483 4483 4483
4477 4477 4481 4478 4478 4477 4477 4478 4478 4478 4479 4477 4483 4483 4483
4477 4477 4484 4478 4478 4477 4477 4478 4478 4478 4479 4477 4483 4483 4483
4477 4477 4481 4478 4478 4477 4477 4478 4478 4478 4479 4477 4480 4481 4481
4483 4483 4483 4478 4478 4477 4477 4478 4478 4478 4479 4477 4483 4481 4481
4482 4482 4481 4478 4478 4477 4477 4478 4478 4478 4479 4477 4483 4483 4483
4483 4483 4484 4478 4478 4477 4477 4478 4478 4478 4479 4477 4481 4482 4482
4477 4477 4483 4478 4478 4477 4477 4478 4478 4478 4479 4477 4481 4481 4481
4477 4477 4483 4478 4478 4477 4477 4478 4478 4478 4479 4477 4482 4482 4482
4483 4483 4483 4478 4478 4477 4477 4478 4478 4478 4479 4477 4481 4481 4481
4477 4477 4481 4478 4478 4477 4477 4478 4478 4478 4479 4477 4482 4482 4482
4483 4483 4481 4478 4478 4477 4477 4478 4478 4478 4479 4477 4482 4482 4482
4477 4477 4481 4478 4478 4477 4477 4478 4478 4478 4479 4477 4480 4481 4481
4482 4482 4484 4478 4478 4477 4477 4478 4478 4478 4479 4477 4481 4481 4481
4482 4482 4481 4478 4478 4477 4477 4478 4478 4478 4479 4477 4481 4482 4482

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 4479 bytes 100%
1,000 4478 bytes -1 byte 100%
10,000 4477 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
4477 bytes – (0.00%)
4477 bytes – (0.00%)
4499 bytes +22 bytes (+0.49%) +22 bytes
4527 bytes +50 bytes (+1.12%) +50 bytes
4542 bytes +65 bytes (+1.45%) +65 bytes
4576 bytes +99 bytes (+2.21%) +99 bytes
4613 bytes +136 bytes (+3.04%) +136 bytes
4644 bytes +167 bytes (+3.73%) +167 bytes
4675 bytes +198 bytes (+4.42%) +198 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 3970 bytes -507 bytes (-11.32%)
Brotli (Wikipedia) brotli brotli -q 11 4171 bytes -306 bytes (-6.83%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 4388 bytes -89 bytes (-1.99%)
ZSTD (Wikipedia) zstd zstd -19 4494 bytes +17 bytes (+0.38%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 4500 bytes +23 bytes (+0.51%)
LZMA2 (Wikipedia) xz xz -9 4560 bytes +83 bytes (+1.85%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 4824 bytes +347 bytes (+7.75%)

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 2017.
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