Choose a version:
35% The original file has 29079 bytes (28.4k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 10260 bytes (10.0k, 35%).

After GZIP compression these minified files vary in size:
Boot
  3922 bytes (3.8k)
CDN, direct link
Baidu
  3545 bytes (3.5k)
CDN, direct link
gzip -6 (default)
  3538 bytes (3.5k)
local copy
gzip -9
  3537 bytes (3.5k)
local copy
unpkg
  3533 bytes (3.5k)
CDN, direct link
cdnjs
  3529 bytes (3.4k)
CDN, direct link
cdnhttps
  3514 bytes (3.4k)
CDN, direct link
libdeflate -12
  3445 bytes (3.4k)
local copy
7zip -mx=9 -tgzip
  3443 bytes (3.4k)
local copy
kzip -s0 -rn -b1
  3439 bytes (3.4k)
local copy
pigz -11 -n
  3433 bytes (3.4k)
local copy
Zopfli
  3430 bytes (3.3k)
local copy
Zopfli (defluff)
  3429 bytes (3.3k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.1.7.min.js

You will automatically get the smallest UnderscoreJS 1.1.7 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 84 bytes by using my UnderscoreJS 1.1.7 Zopfli version instead of the best available CDN (2.45% smaller than cdnhttps, 3430 vs. 3514 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 --mls64 --bsr18 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 64  --mls64
block splitting recursion 18  --bsr18
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

Even Smaller Files Thanks To Defluff

Zopfli's output can be further optimized by the defluff tool.
In this particular case, defluff saves 1 more byte (3429 bytes).

Verify file integrity

After decompression, my uncompressed files are identical to the original ones:

MD5:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.1.7/underscore-min.js --location | md5sum
6f6cfe9c37413b6951178ff862e06ffc  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.1.7.min.zopfli.js.gz | md5sum
6f6cfe9c37413b6951178ff862e06ffc  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.1.7/underscore-min.js --location | sha1sum
81afb4cd3755671fea5e74d7118d7d460c475308  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.1.7.min.zopfli.js.gz | sha1sum
81afb4cd3755671fea5e74d7118d7d460c475308  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 3922 bytes 6f6cfe9c37413b6951178ff862e06ffc (invalid)
unpkg 3533 bytes 6f6cfe9c37413b6951178ff862e06ffc July 11, 2016 @ 16:49
cdnjs 3529 bytes 6f6cfe9c37413b6951178ff862e06ffc (invalid)
cdnhttps 3514 bytes 6f6cfe9c37413b6951178ff862e06ffc (invalid)

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 3545 bytes e1cb863a7854988cacf4371b5a4d02d9 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
3430 bytes -1 byte zopfli --i10000 --mls64 --bsr18 --lazy --ohh February 2, 2016 @ 22:11
3431 bytes -1 byte zopfli --i1000 --mls1024 --bsr6 --lazy --ohh February 2, 2016 @ 19:46
3432 bytes zopfli --i100 --mls32 --bsr17 --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 @ 16:49.

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
3433 3433 3432 3432 3432 3432 3432 3432 3432 3433 3433 3433 3431 3432 3432
3433 3433 3432 3432 3432 3432 3431 3432 3432 3433 3433 3433 3431 3431 3431
3432 3432 3433 3432 3431 3433 3432 3432 3431 3431 3431 3431 3430 3430 3430
3433 3433 3431 3431 3432 3430 3431 3432 3432 3431 3431 3432 3431 3431 3431
3432 3432 3432 3431 3432 3431 3431 3432 3430 3431 3431 3432 3431 3431 3431
3432 3432 3431 3430 3432 3430 3431 3432 3431 3431 3431 3433 3431 3431 3431
3432 3432 3431 3431 3432 3430 3431 3432 3431 3431 3431 3432 3431 3431 3431
3432 3432 3431 3432 3432 3433 3433 3432 3431 3432 3431 3430 3430 3430 3430
3432 3432 3431 3432 3432 3433 3432 3432 3432 3432 3431 3432 3430 3430 3430
3432 3432 3431 3432 3432 3432 3431 3432 3432 3431 3431 3431 3430 3430 3430
3432 3432 3431 3431 3431 3432 3432 3432 3431 3432 3432 3430 3431 3431 3431
3432 3432 3431 3431 3432 3433 3432 3432 3430 3431 3431 3431 3430 3430 3430
3432 3432 3431 3431 3432 3433 3431 3432 3430 3431 3431 3433 3430 3430 3430
3432 3432 3431 3432 3431 3433 3432 3431 3431 3431 3431 3431 3433 3430 3430
3432 3432 3433 3432 3432 3430 3432 3431 3431 3431 3431 3432 3431 3431 3431
3432 3432 3431 3431 3431 3433 3433 3432 3430 3431 3432 3431 3431 3431 3431
3432 3432 3431 3432 3432 3430 3432 3432 3431 3431 3431 3431 3431 3431 3431
3433 3433 3431 3431 3431 3433 3432 3432 3431 3431 3432 3432 3431 3431 3431
3433 3433 3431 3431 3431 3430 3431 3432 3432 3432 3431 3431 3431 3431 3431
3432 3432 3432 3431 3431 3430 3433 3431 3432 3431 3431 3430 3430 3430 3430
3432 3432 3432 3431 3431 3430 3432 3432 3432 3431 3431 3432 3433 3430 3430
3432 3432 3433 3431 3432 3432 3432 3431 3432 3432 3431 3432 3430 3430 3430
3432 3432 3431 3433 3432 3432 3432 3431 3432 3431 3431 3430 3431 3431 3431

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 3432 bytes 100%
1,000 3431 bytes -1 byte 100%
10,000 3430 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
3439 bytes +9 bytes (+0.26%)
3439 bytes +9 bytes (+0.26%)
3461 bytes +31 bytes (+0.90%) +22 bytes
3458 bytes +28 bytes (+0.82%) +19 bytes
3484 bytes +54 bytes (+1.57%) +45 bytes
3519 bytes +89 bytes (+2.59%) +80 bytes
3551 bytes +121 bytes (+3.53%) +112 bytes
3578 bytes +148 bytes (+4.31%) +139 bytes
3611 bytes +181 bytes (+5.28%) +172 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 3082 bytes -348 bytes (-10.15%)
Brotli (Wikipedia) brotli brotli -q 11 3165 bytes -265 bytes (-7.73%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 3394 bytes -36 bytes (-1.05%)
ZSTD (Wikipedia) zstd zstd -19 3456 bytes +26 bytes (+0.76%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 3469 bytes +39 bytes (+1.14%)
LZMA2 (Wikipedia) xz xz -9 3536 bytes +106 bytes (+3.09%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 3951 bytes +521 bytes (+15.19%)

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