Choose a version:
37% The original file has 20652 bytes (20.2k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 7595 bytes (7.4k, 37%).

After GZIP compression these minified files vary in size:
Boot
  2702 bytes (2.6k)
CDN
gzip -6 (default)
  2447 bytes (2.4k)
local copy
gzip -9
  2445 bytes (2.4k)
local copy
Baidu
  2441 bytes (2.4k)
CDN
cdnjs
  2432 bytes (2.4k)
CDN
cdnhttps
  2423 bytes (2.4k)
CDN
7zip -mx=9 -tgzip
  2375 bytes (2.3k)
local copy
libdeflate -12
  2374 bytes (2.3k)
local copy
kzip -s0 -rn -b0
  2372 bytes (2.3k)
local copy
pigz -11 -n
  2372 bytes (2.3k)
local copy
Zopfli
  2369 bytes (2.3k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.7.min.js

You will automatically get the smallest UnderscoreJS 0.4.7 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 54 bytes by using my UnderscoreJS 0.4.7 Zopfli version instead of the best available CDN (2.28% smaller than cdnhttps, 2369 vs. 2423 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 --mls32768 --bsr10 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 1000  --i1000
maximum blocks 8  --mb8
maximum length score 32768  --mls32768
block splitting recursion 10  --bsr10
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/0.4.7/underscore-min.js --location | md5sum
24ef658fdf19c9b776d88433b826ca2e  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.7.min.zopfli.js.gz | md5sum
24ef658fdf19c9b776d88433b826ca2e  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.4.7/underscore-min.js --location | sha1sum
eb4815b3ecf807789c85bf826b20a42fc160b6d0  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.7.min.zopfli.js.gz | sha1sum
eb4815b3ecf807789c85bf826b20a42fc160b6d0  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 2702 bytes 24ef658fdf19c9b776d88433b826ca2e (invalid)
cdnjs 2432 bytes 24ef658fdf19c9b776d88433b826ca2e (invalid)
cdnhttps 2423 bytes 24ef658fdf19c9b776d88433b826ca2e (invalid)

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 2441 bytes 7108b98977bda92b7edc6ffb5160dc18 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
2369 bytes -2 bytes zopfli --i1000 --mls32768 --bsr10 --lazy --ohh February 2, 2016 @ 20:58
2371 bytes zopfli --i100 --mls32768 --bsr10 --lazy --ohh February 2, 2016 @ 20:57

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 @ 14:35.

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
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369
2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369 2369

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 2370 bytes 100%
1,000 2369 bytes -1 byte 100%
10,000 2369 bytes 100%
100,000 2369 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
2372 bytes +3 bytes (+0.13%)
2373 bytes +4 bytes (+0.17%) +1 byte
2400 bytes +31 bytes (+1.31%) +28 bytes
2424 bytes +55 bytes (+2.32%) +52 bytes
2458 bytes +89 bytes (+3.76%) +86 bytes
2481 bytes +112 bytes (+4.73%) +109 bytes
2512 bytes +143 bytes (+6.04%) +140 bytes
2547 bytes +178 bytes (+7.51%) +175 bytes
2578 bytes +209 bytes (+8.82%) +206 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 2140 bytes -229 bytes (-9.67%)
Brotli (Wikipedia) brotli brotli -q 11 2185 bytes -184 bytes (-7.77%)
ZSTD (Wikipedia) zstd zstd -19 2387 bytes +18 bytes (+0.76%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 2453 bytes +84 bytes (+3.55%)
LZMA2 (Wikipedia) xz xz -9 2468 bytes +99 bytes (+4.18%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 2621 bytes +252 bytes (+10.64%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 3046 bytes +677 bytes (+28.58%)

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