Choose a version:
37% The original file has 21601 bytes (21.1k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 7943 bytes (7.8k, 37%).

After GZIP compression these minified files vary in size:
Boot
  2893 bytes (2.8k)
CDN, direct link
gzip -6 (default)
  2625 bytes (2.6k)
local copy
gzip -9
  2624 bytes (2.6k)
local copy
Baidu
  2620 bytes (2.6k)
CDN, direct link
cdnjs
  2608 bytes (2.5k)
CDN, direct link
cdnhttps
  2601 bytes (2.5k)
CDN, direct link
libdeflate -12
  2554 bytes (2.5k)
local copy
7zip -mx=9 -tgzip
  2553 bytes (2.5k)
local copy
pigz -11 -n
  2550 bytes (2.5k)
local copy
kzip -s0 -rn -b1
  2549 bytes (2.5k)
local copy
Zopfli
  2548 bytes (2.5k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.1.min.js

You will automatically get the smallest UnderscoreJS 0.5.1 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 53 bytes by using my UnderscoreJS 0.5.1 Zopfli version instead of the best available CDN (2.08% smaller than cdnhttps, 2548 vs. 2601 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 --mls64 --bsr2 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 1000  --i1000
maximum blocks 8  --mb8
maximum length score 64  --mls64
block splitting recursion 2  --bsr2
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/0.5.1/underscore-min.js --location | md5sum
ad16532134e29bb985e5427af25745ee  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.1.min.zopfli.js.gz | md5sum
ad16532134e29bb985e5427af25745ee  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.5.1/underscore-min.js --location | sha1sum
f1cb62fb02a78ec5c32c20f50e1e523b2b18ede0  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.1.min.zopfli.js.gz | sha1sum
f1cb62fb02a78ec5c32c20f50e1e523b2b18ede0  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 2893 bytes ad16532134e29bb985e5427af25745ee (invalid)
cdnjs 2608 bytes ad16532134e29bb985e5427af25745ee (invalid)
cdnhttps 2601 bytes ad16532134e29bb985e5427af25745ee (invalid)

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 2620 bytes 3f6e7804f166f4f45fe3be1531c3fdf6 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
2548 bytes -1 byte zopfli --i1000 --mls64 --bsr2 --lazy --ohh February 2, 2016 @ 20:58
2549 bytes zopfli --i100 --mls16 --bsr14 --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 @ 14:41.

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
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548
2548 2548 2548 2549 2548 2548 2548 2548 2548 2548 2549 2548 2548 2548 2548

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 2549 bytes 100%
1,000 2548 bytes -1 byte 100%
10,000 2548 bytes 100%
100,000 2548 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
2549 bytes +1 byte (+0.04%)
2549 bytes +1 byte (+0.04%)
2577 bytes +29 bytes (+1.14%) +28 bytes
2610 bytes +62 bytes (+2.43%) +61 bytes
2622 bytes +74 bytes (+2.90%) +73 bytes
2650 bytes +102 bytes (+4.00%) +101 bytes
2679 bytes +131 bytes (+5.14%) +130 bytes
2703 bytes +155 bytes (+6.08%) +154 bytes
2742 bytes +194 bytes (+7.61%) +193 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 2290 bytes -258 bytes (-10.13%)
Brotli (Wikipedia) brotli brotli -q 11 2347 bytes -201 bytes (-7.89%)
ZSTD (Wikipedia) zstd zstd -19 2562 bytes +14 bytes (+0.55%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 2617 bytes +69 bytes (+2.71%)
LZMA2 (Wikipedia) xz xz -9 2644 bytes +96 bytes (+3.77%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 2793 bytes +245 bytes (+9.62%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 3184 bytes +636 bytes (+24.96%)

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