Choose a version:
37% The original file has 19261 bytes (18.8k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 7146 bytes (7.0k, 37%).

After GZIP compression these minified files vary in size:
Boot
  2516 bytes (2.5k)
CDN
gzip -6 (default)
  2278 bytes (2.2k)
local copy
gzip -9
  2278 bytes (2.2k)
local copy
Baidu
  2273 bytes (2.2k)
CDN
cdnjs
  2260 bytes (2.2k)
CDN
cdnhttps
  2254 bytes (2.2k)
CDN
libdeflate -12
  2216 bytes (2.2k)
local copy
7zip -mx=9 -tgzip
  2214 bytes (2.2k)
local copy
kzip -s0 -rn -b1
  2212 bytes (2.2k)
local copy
pigz -11 -n
  2211 bytes (2.2k)
local copy
Zopfli
  2207 bytes (2.2k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.5.min.js

You will automatically get the smallest UnderscoreJS 0.4.5 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 47 bytes by using my UnderscoreJS 0.4.5 Zopfli version instead of the best available CDN (2.13% smaller than cdnhttps, 2207 vs. 2254 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 --mls2 --bsr4 --lazy --ohh

(found February 2, 2016)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 2  --mls2
block splitting recursion 4  --bsr4
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/0.4.5/underscore-min.js --location | md5sum
7b8134844add60af3f686c0f3a7d0b06  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.5.min.zopfli.js.gz | md5sum
7b8134844add60af3f686c0f3a7d0b06  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.4.5/underscore-min.js --location | sha1sum
7a0c1331b6b75b8f36f2812146f5f43d406334c8  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.4.5.min.zopfli.js.gz | sha1sum
7a0c1331b6b75b8f36f2812146f5f43d406334c8  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 2516 bytes 7b8134844add60af3f686c0f3a7d0b06 (invalid)
cdnjs 2260 bytes 7b8134844add60af3f686c0f3a7d0b06 (invalid)
cdnhttps 2254 bytes 7b8134844add60af3f686c0f3a7d0b06 (invalid)

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 2273 bytes 8ec18a70d407996c02d3284e762d22f1 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
2207 bytes -2 bytes zopfli --i10000 --mls2 --bsr4 --lazy --ohh February 2, 2016 @ 23:42
2209 bytes zopfli --i100 --mls16384 --bsr13 --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
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208
2207 2207 2207 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208 2208

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 2209 bytes 100%
1,000 2209 bytes 100%
10,000 2207 bytes -2 bytes 100%
100,000 2207 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
2213 bytes +6 bytes (+0.27%) +1 byte
2212 bytes +5 bytes (+0.23%)
2238 bytes +31 bytes (+1.40%) +26 bytes
2264 bytes +57 bytes (+2.58%) +52 bytes
2299 bytes +92 bytes (+4.17%) +87 bytes
2334 bytes +127 bytes (+5.75%) +122 bytes
2356 bytes +149 bytes (+6.75%) +144 bytes
2380 bytes +173 bytes (+7.84%) +168 bytes
2414 bytes +207 bytes (+9.38%) +202 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 1997 bytes -210 bytes (-9.52%)
Brotli (Wikipedia) brotli brotli -q 11 2025 bytes -182 bytes (-8.25%)
ZSTD (Wikipedia) zstd zstd -19 2226 bytes +19 bytes (+0.86%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 2292 bytes +85 bytes (+3.85%)
LZMA2 (Wikipedia) xz xz -9 2304 bytes +97 bytes (+4.40%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 2445 bytes +238 bytes (+10.78%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 2915 bytes +708 bytes (+32.08%)

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