Choose a version:
37% The original file has 22299 bytes (21.8k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 8194 bytes (8.0k, 37%).

After GZIP compression these minified files vary in size:
Boot
  2960 bytes (2.9k)
CDN
gzip -6 (default)
  2682 bytes (2.6k)
local copy
gzip -9
  2681 bytes (2.6k)
local copy
Baidu
  2678 bytes (2.6k)
CDN
cdnjs
  2666 bytes (2.6k)
CDN
cdnhttps
  2658 bytes (2.6k)
CDN
libdeflate -12
  2605 bytes (2.5k)
local copy
7zip -mx=9 -tgzip
  2602 bytes (2.5k)
local copy
kzip -s0 -rn -b1
  2599 bytes (2.5k)
local copy
pigz -11 -n
  2599 bytes (2.5k)
local copy
Zopfli
  2595 bytes (2.5k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.4.min.js

You will automatically get the smallest UnderscoreJS 0.5.4 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 63 bytes by using my UnderscoreJS 0.5.4 Zopfli version instead of the best available CDN (2.43% smaller than cdnhttps, 2595 vs. 2658 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 --mls8 --bsr7 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 8  --mls8
block splitting recursion 7  --bsr7
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/0.5.4/underscore-min.js --location | md5sum
426f6be73c1adb6234fa85520b8c4cf6  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.4.min.zopfli.js.gz | md5sum
426f6be73c1adb6234fa85520b8c4cf6  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.5.4/underscore-min.js --location | sha1sum
7ab2ea271ad5bd3de9e0f17ded546a25fc6d25fc  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.4.min.zopfli.js.gz | sha1sum
7ab2ea271ad5bd3de9e0f17ded546a25fc6d25fc  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 2960 bytes 426f6be73c1adb6234fa85520b8c4cf6 (invalid)
cdnjs 2666 bytes 426f6be73c1adb6234fa85520b8c4cf6 (invalid)
cdnhttps 2658 bytes 426f6be73c1adb6234fa85520b8c4cf6 (invalid)

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 2678 bytes dc17bde7ba8728cb1319be153930eb60 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
2595 bytes -1 byte zopfli --i10000 --mls8 --bsr7 --lazy --ohh February 2, 2016 @ 23:28
2596 bytes -1 byte zopfli --i1000 --mls4096 --bsr20 --lazy --ohh February 2, 2016 @ 20:59
2597 bytes zopfli --i100 --mls16384 --bsr8 --lazy --ohh February 2, 2016 @ 20:58

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 @ 14:39.

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
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595
2595 2595 2595 2595 2595 2595 2595 2596 2595 2595 2596 2595 2595 2595 2595

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 2597 bytes 100%
1,000 2596 bytes -1 byte 100%
10,000 2595 bytes -1 byte 100%
100,000 2595 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
2599 bytes +4 bytes (+0.15%)
2599 bytes +4 bytes (+0.15%)
2622 bytes +27 bytes (+1.04%) +23 bytes
2656 bytes +61 bytes (+2.35%) +57 bytes
2669 bytes +74 bytes (+2.85%) +70 bytes
2696 bytes +101 bytes (+3.89%) +97 bytes
2723 bytes +128 bytes (+4.93%) +124 bytes
2758 bytes +163 bytes (+6.28%) +159 bytes
2793 bytes +198 bytes (+7.63%) +194 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 2325 bytes -270 bytes (-10.40%)
Brotli (Wikipedia) brotli brotli -q 11 2399 bytes -196 bytes (-7.55%)
ZSTD (Wikipedia) zstd zstd -19 2609 bytes +14 bytes (+0.54%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 2646 bytes +51 bytes (+1.97%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 2677 bytes +82 bytes (+3.16%)
LZMA2 (Wikipedia) xz xz -9 2696 bytes +101 bytes (+3.89%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 3221 bytes +626 bytes (+24.12%)

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 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