Choose a version:
33% The original file has 40966 bytes (40.0k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 13582 bytes (13.3k, 33%).

After GZIP compression these minified files vary in size:
jsdelivr
  5193 bytes (5.1k)
CDN, direct link
Boot
  5190 bytes (5.1k)
CDN, direct link
gzip -6 (default)
  4677 bytes (4.6k)
local copy
Baidu
  4671 bytes (4.6k)
CDN, direct link
cdnjs
  4669 bytes (4.6k)
CDN, direct link
gzip -9
  4659 bytes (4.5k)
local copy
cdnhttps
  4653 bytes (4.5k)
CDN, direct link
unpkg
  4650 bytes (4.5k)
CDN, direct link
7zip -mx=9 -tgzip
  4541 bytes (4.4k)
local copy
libdeflate -12
  4536 bytes (4.4k)
local copy
kzip -s0 -rn -b1
  4531 bytes (4.4k)
local copy
pigz -11 -n
  4527 bytes (4.4k)
local copy
Zopfli
  4521 bytes (4.4k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.4.2.min.js

You will automatically get the smallest UnderscoreJS 1.4.2 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 129 bytes by using my UnderscoreJS 1.4.2 Zopfli version instead of the best available CDN (2.85% smaller than unpkg, 4521 vs. 4650 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 --i100000 --mb8 --mls1024 --bsr14 --lazy --ohh

(found November 23, 2015)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 1024  --mls1024
block splitting recursion 14  --bsr14
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/1.4.2/underscore-min.js --location | md5sum
969feaaba18c01015451501674545f1f  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.4.2.min.zopfli.js.gz | md5sum
969feaaba18c01015451501674545f1f  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.4.2/underscore-min.js --location | sha1sum
507452bd3e2df95b8945a1e5b251595c11e113b0  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.4.2.min.zopfli.js.gz | sha1sum
507452bd3e2df95b8945a1e5b251595c11e113b0  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 5190 bytes 969feaaba18c01015451501674545f1f (invalid)
cdnjs 4669 bytes 969feaaba18c01015451501674545f1f (invalid)
cdnhttps 4653 bytes 969feaaba18c01015451501674545f1f December 24, 2015 @ 07:36
unpkg 4650 bytes 969feaaba18c01015451501674545f1f July 11, 2016 @ 16:49

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
jsdelivr 5193 bytes 90531ffe720f6b9306ac02aaf9606a00 only whitespaces differ (invalid)
Baidu 4671 bytes 90531ffe720f6b9306ac02aaf9606a00 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:

1.8.3, 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
4521 bytes -1 byte zopfli --i100000 --mls1024 --bsr14 --lazy --ohh November 23, 2015 @ 18:28
4522 bytes -1 byte zopfli --i10000 --mls1024 --bsr14 --lazy --ohh September 8, 2015 @ 14:09
4523 bytes -2 bytes zopfli --i1000 --mls1024 --bsr14 --lazy --ohh September 8, 2015 @ 10:50
4525 bytes zopfli --i100 --mls4096 --bsr16 --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 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
4532 4532 4535 4535 4525 4535 4532 4532 4525 4524 4525 4525 4524 4524 4524
4530 4530 4530 4529 4526 4527 4527 4528 4524 4524 4524 4525 4524 4524 4524
4528 4528 4525 4526 4527 4525 4524 4532 4525 4525 4524 4524 4524 4524 4524
4526 4526 4526 4526 4526 4526 4526 4526 4524 4529 4523 4524 4525 4525 4525
4529 4529 4530 4526 4527 4526 4528 4529 4524 4528 4525 4524 4525 4524 4524
4526 4526 4526 4527 4526 4523 4526 4528 4524 4525 4526 4525 4525 4525 4525
4527 4527 4526 4526 4536 4530 4527 4524 4525 4524 4523 4525 4527 4526 4526
4528 4528 4532 4536 4526 4527 4527 4529 4524 4525 4523 4524 4524 4525 4525
4525 4525 4526 4536 4536 4526 4527 4525 4524 4524 4524 4524 4525 4525 4525
4526 4526 4524 4526 4527 4526 4524 4529 4524 4524 4523 4525 4525 4525 4525
4528 4528 4525 4526 4524 4524 4527 4524 4524 4521 4526 4524 4524 4524 4524
4526 4526 4525 4526 4536 4526 4527 4529 4524 4526 4524 4525 4525 4524 4524
4526 4526 4526 4525 4526 4537 4525 4525 4526 4524 4526 4524 4525 4524 4524
4526 4526 4528 4527 4527 4526 4527 4529 4524 4525 4523 4524 4525 4525 4525
4525 4525 4526 4527 4527 4535 4524 4524 4526 4526 4525 4525 4525 4524 4524
4526 4526 4527 4525 4526 4523 4527 4530 4524 4525 4525 4523 4524 4524 4524
4526 4526 4526 4525 4527 4524 4526 4529 4525 4525 4523 4525 4524 4524 4524
4526 4526 4526 4527 4526 4526 4527 4529 4524 4525 4523 4525 4525 4528 4528
4525 4525 4525 4527 4526 4529 4527 4525 4524 4525 4523 4525 4525 4525 4525
4525 4525 4525 4526 4527 4526 4525 4529 4524 4524 4523 4525 4525 4524 4524
4528 4528 4528 4525 4537 4524 4529 4526 4523 4524 4524 4525 4525 4526 4526
4526 4526 4526 4535 4524 4524 4532 4527 4524 4523 4524 4524 4525 4524 4524
4526 4526 4526 4526 4526 4537 4527 4524 4524 4526 4524 4524 4525 4525 4525

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 4524 bytes 100%
1,000 4523 bytes -1 byte 100%
10,000 4522 bytes -1 byte 100%
100,000 4521 bytes -1 byte 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
4531 bytes +10 bytes (+0.22%)
4531 bytes +10 bytes (+0.22%)
4549 bytes +28 bytes (+0.62%) +18 bytes
4548 bytes +27 bytes (+0.60%) +17 bytes
4582 bytes +61 bytes (+1.35%) +51 bytes
4616 bytes +95 bytes (+2.10%) +85 bytes
4653 bytes +132 bytes (+2.92%) +122 bytes
4680 bytes +159 bytes (+3.52%) +149 bytes
4713 bytes +192 bytes (+4.25%) +182 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 4035 bytes -486 bytes (-10.75%)
Brotli (Wikipedia) brotli brotli -q 11 4196 bytes -325 bytes (-7.19%)
ZSTD (Wikipedia) zstd zstd -19 4541 bytes +20 bytes (+0.44%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 4557 bytes +36 bytes (+0.80%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 4568 bytes +47 bytes (+1.04%)
LZMA2 (Wikipedia) xz xz -9 4608 bytes +87 bytes (+1.92%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 4888 bytes +367 bytes (+8.12%)

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