Choose a version:
36% The original file has 27738 bytes (27.1k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 9963 bytes (9.7k, 36%).

After GZIP compression these minified files vary in size:
Boot
  3757 bytes (3.7k)
CDN
Baidu
  3408 bytes (3.3k)
CDN
gzip -6 (default)
  3406 bytes (3.3k)
local copy
gzip -9
  3404 bytes (3.3k)
local copy
cdnjs
  3394 bytes (3.3k)
CDN
cdnhttps
  3382 bytes (3.3k)
CDN
libdeflate -12
  3315 bytes (3.2k)
local copy
7zip -mx=9 -tgzip
  3312 bytes (3.2k)
local copy
kzip -s0 -rn -b0
  3310 bytes (3.2k)
local copy
pigz -11 -n
  3303 bytes (3.2k)
local copy
Zopfli
  3299 bytes (3.2k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.1.5.min.js

You will automatically get the smallest UnderscoreJS 1.1.5 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 83 bytes by using my UnderscoreJS 1.1.5 Zopfli version instead of the best available CDN (2.52% smaller than cdnhttps, 3299 vs. 3382 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 --i100 --mb8 --mls512 --bsr40 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 100  --i100
maximum blocks 8  --mb8
maximum length score 512  --mls512
block splitting recursion 40  --bsr40
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/1.1.5/underscore-min.js --location | md5sum
21619893ce1bc4029f11c1c6ec1ecabf  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.1.5.min.zopfli.js.gz | md5sum
21619893ce1bc4029f11c1c6ec1ecabf  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.1.5/underscore-min.js --location | sha1sum
f665459959c5167c0905974a62ffe923f2c19813  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.1.5.min.zopfli.js.gz | sha1sum
f665459959c5167c0905974a62ffe923f2c19813  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 3757 bytes 21619893ce1bc4029f11c1c6ec1ecabf (invalid)
cdnjs 3394 bytes 21619893ce1bc4029f11c1c6ec1ecabf (invalid)
cdnhttps 3382 bytes 21619893ce1bc4029f11c1c6ec1ecabf (invalid)

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 3408 bytes d7fc74b91595fdb53ef9c47fdd955802 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
3299 bytes zopfli --i100 --mls512 --bsr40 --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 June 16, 2016 @ 14:40.

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
3299 3299 3299 3299 3300 3300 3311 3300 3300 3300 3300 3301 3301 3301 3301
3301 3301 3300 3300 3301 3302 3301 3302 3301 3304 3301 3300 3300 3300 3300
3302 3302 3301 3301 3300 3301 3301 3300 3301 3304 3299 3301 3299 3299 3299
3300 3300 3306 3300 3300 3301 3301 3299 3299 3299 3301 3300 3299 3299 3299
3300 3300 3306 3299 3300 3300 3307 3300 3300 3300 3300 3302 3299 3299 3299
3300 3300 3300 3300 3300 3300 3300 3299 3301 3301 3304 3300 3299 3299 3299
3302 3302 3302 3300 3300 3300 3301 3300 3299 3304 3301 3300 3301 3299 3299
3302 3302 3302 3300 3300 3311 3300 3301 3299 3299 3299 3301 3299 3299 3299
3300 3300 3302 3300 3300 3301 3300 3302 3299 3301 3301 3300 3299 3299 3299
3299 3299 3302 3300 3300 3300 3301 3301 3299 3304 3299 3301 3299 3299 3299
3300 3300 3300 3300 3300 3300 3300 3300 3301 3299 3302 3300 3299 3299 3299
3307 3307 3306 3300 3300 3308 3301 3301 3299 3301 3299 3301 3301 3299 3299
3302 3302 3300 3300 3300 3300 3301 3300 3299 3301 3302 3301 3299 3299 3299
3301 3301 3302 3300 3300 3300 3300 3301 3299 3301 3299 3300 3301 3299 3299
3301 3301 3307 3299 3300 3299 3300 3301 3299 3304 3303 3301 3299 3299 3299
3300 3300 3301 3300 3300 3299 3301 3300 3299 3304 3299 3300 3299 3299 3299
3300 3300 3301 3300 3300 3300 3301 3301 3299 3304 3299 3301 3299 3299 3299
3300 3300 3300 3300 3300 3301 3300 3300 3299 3301 3299 3301 3301 3301 3301
3301 3301 3300 3300 3300 3301 3301 3301 3299 3301 3300 3301 3299 3299 3299
3302 3302 3304 3299 3300 3300 3300 3300 3299 3304 3302 3301 3300 3299 3299
3300 3300 3300 3301 3300 3300 3301 3300 3299 3304 3302 3301 3300 3300 3300
3299 3299 3301 3300 3300 3300 3301 3300 3301 3301 3299 3299 3300 3300 3300
3302 3302 3301 3299 3300 3301 3301 3300 3299 3301 3299 3300 3301 3301 3301

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 3299 bytes 100%
1,000 3299 bytes 100%
10,000 3299 bytes 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
3310 bytes +11 bytes (+0.33%)
3310 bytes +11 bytes (+0.33%)
3330 bytes +31 bytes (+0.94%) +20 bytes
3331 bytes +32 bytes (+0.97%) +21 bytes
3362 bytes +63 bytes (+1.91%) +52 bytes
3389 bytes +90 bytes (+2.73%) +79 bytes
3422 bytes +123 bytes (+3.73%) +112 bytes
3459 bytes +160 bytes (+4.85%) +149 bytes
3494 bytes +195 bytes (+5.91%) +184 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 2967 bytes -332 bytes (-10.06%)
Brotli (Wikipedia) brotli brotli -q 11 3061 bytes -238 bytes (-7.21%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 3289 bytes -10 bytes (-0.30%)
ZSTD (Wikipedia) zstd zstd -19 3327 bytes +28 bytes (+0.85%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 3357 bytes +58 bytes (+1.76%)
LZMA2 (Wikipedia) xz xz -9 3412 bytes +113 bytes (+3.43%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 3857 bytes +558 bytes (+16.91%)

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