Choose a version:
36% The original file has 26968 bytes (26.3k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 9669 bytes (9.4k, 36%).

After GZIP compression these minified files vary in size:
Boot
  3667 bytes (3.6k)
CDN
Baidu
  3328 bytes (3.3k)
CDN
gzip -6 (default)
  3327 bytes (3.2k)
local copy
gzip -9
  3324 bytes (3.2k)
local copy
unpkg
  3321 bytes (3.2k)
CDN
cdnjs
  3316 bytes (3.2k)
CDN
cdnhttps
  3303 bytes (3.2k)
CDN
libdeflate -12
  3237 bytes (3.2k)
local copy
7zip -mx=9 -tgzip
  3233 bytes (3.2k)
local copy
kzip -s0 -rn -b1
  3231 bytes (3.2k)
local copy
pigz -11 -n
  3226 bytes (3.2k)
local copy
Zopfli
  3220 bytes (3.1k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.1.4.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 1.1.4 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 83 bytes by using my UnderscoreJS 1.1.4 Zopfli version instead of the best available CDN (2.58% smaller than cdnhttps, 3220 vs. 3303 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 --mls256 --bsr23 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 256  --mls256
block splitting recursion 23  --bsr23
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/1.1.4/underscore-min.js --location | md5sum
bb6754eab891e4c62e78b77f6652b54d  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.1.4.min.zopfli.js.gz | md5sum
bb6754eab891e4c62e78b77f6652b54d  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.1.4/underscore-min.js --location | sha1sum
2de2ecea5ba12e7235ffb44d6892cc77fe6f4abe  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.1.4.min.zopfli.js.gz | sha1sum
2de2ecea5ba12e7235ffb44d6892cc77fe6f4abe  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 3667 bytes bb6754eab891e4c62e78b77f6652b54d (invalid)
unpkg 3321 bytes bb6754eab891e4c62e78b77f6652b54d July 11, 2016 @ 16:49
cdnjs 3316 bytes bb6754eab891e4c62e78b77f6652b54d (invalid)
cdnhttps 3303 bytes bb6754eab891e4c62e78b77f6652b54d (invalid)

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 3328 bytes 178201c529411d163a6801134d2d45fc 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:

1.9.1, 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
3220 bytes -1 byte zopfli --i10000 --mls256 --bsr23 --lazy --ohh February 2, 2016 @ 21:49
3221 bytes -1 byte zopfli --i1000 --mls16384 --bsr9 --lazy --ohh February 2, 2016 @ 19:46
3222 bytes zopfli --i100 --mls16384 --bsr11 --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
3222 3222 3222 3222 3221 3221 3221 3221 3221 3222 3222 3224 3223 3223 3223
3222 3222 3222 3221 3229 3222 3223 3224 3224 3224 3222 3221 3221 3221 3221
3229 3229 3231 3230 3221 3221 3223 3222 3222 3224 3224 3224 3222 3222 3222
3221 3221 3221 3221 3228 3229 3223 3223 3222 3223 3224 3223 3222 3222 3222
3221 3221 3223 3222 3221 3221 3221 3221 3221 3222 3222 3223 3223 3223 3223
3221 3221 3223 3221 3221 3221 3223 3221 3222 3223 3224 3222 3221 3221 3221
3229 3229 3221 3222 3221 3221 3221 3222 3221 3222 3222 3222 3221 3221 3221
3230 3230 3231 3221 3221 3221 3221 3221 3221 3222 3224 3224 3221 3221 3221
3221 3221 3222 3221 3221 3221 3223 3221 3222 3223 3222 3223 3221 3221 3221
3221 3221 3221 3223 3221 3223 3221 3221 3222 3222 3223 3223 3222 3222 3222
3221 3221 3221 3221 3230 3229 3223 3221 3222 3224 3222 3223 3223 3223 3223
3222 3222 3221 3222 3221 3221 3223 3223 3222 3221 3222 3223 3223 3223 3223
3222 3222 3221 3221 3221 3221 3221 3221 3222 3222 3222 3223 3221 3221 3221
3222 3222 3221 3223 3221 3221 3221 3222 3221 3222 3223 3224 3223 3223 3223
3221 3221 3221 3221 3221 3221 3221 3221 3222 3223 3223 3221 3223 3223 3223
3221 3221 3221 3221 3221 3221 3221 3221 3221 3222 3222 3221 3221 3221 3221
3221 3221 3221 3221 3221 3221 3221 3221 3222 3222 3223 3223 3221 3221 3221
3221 3221 3221 3221 3222 3222 3221 3221 3222 3221 3223 3222 3221 3221 3221
3222 3222 3222 3221 3222 3221 3220 3220 3223 3222 3222 3223 3222 3222 3222
3222 3222 3221 3221 3221 3229 3221 3220 3222 3221 3223 3224 3221 3221 3221
3221 3221 3221 3221 3221 3221 3220 3221 3222 3222 3223 3223 3221 3221 3221
3222 3222 3221 3221 3221 3222 3221 3220 3222 3221 3222 3223 3221 3221 3221
3221 3221 3221 3221 3221 3220 3222 3221 3222 3222 3223 3223 3221 3221 3221

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 3222 bytes 100%
1,000 3221 bytes -1 byte 100%
10,000 3220 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
3231 bytes +11 bytes (+0.34%)
3231 bytes +11 bytes (+0.34%)
3253 bytes +33 bytes (+1.02%) +22 bytes
3252 bytes +32 bytes (+0.99%) +21 bytes
3281 bytes +61 bytes (+1.89%) +50 bytes
3315 bytes +95 bytes (+2.95%) +84 bytes
3343 bytes +123 bytes (+3.82%) +112 bytes
3376 bytes +156 bytes (+4.84%) +145 bytes
3411 bytes +191 bytes (+5.93%) +180 bytes

Non-DEFLATE Algorithms

Archivers based on completely different compression algorithms often produce superior results.
Unfortunately, browsers only support gzip compression at the moment.
However, support for Brotli is constantly growing - but your browser doesn't support it.
Algorithm Program Parameters Size Compared To Best Zopfli
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 2907 bytes -313 bytes (-9.72%)
Brotli (Wikipedia) brotli brotli -q 11 2971 bytes -249 bytes (-7.73%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 3190 bytes -30 bytes (-0.93%)
Zstandard (Wikipedia) zstd zstd -19 3240 bytes +20 bytes (+0.62%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 3289 bytes +69 bytes (+2.14%)
LZMA2 (Wikipedia) xz xz -9 3328 bytes +108 bytes (+3.35%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 3799 bytes +579 bytes (+17.98%)

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