Choose a version:
31% The original file has 58324 bytes (57.0k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 18070 bytes (17.6k, 31%).

After GZIP compression these minified files vary in size:
unpkg
  7289 bytes (7.1k)
CDN
gzip -6 (default)
  6461 bytes (6.3k)
local copy
gzip -9
  6453 bytes (6.3k)
local copy
7zip -mx=9 -tgzip
  6261 bytes (6.1k)
local copy
libdeflate -12
  6257 bytes (6.1k)
local copy
kzip -s0 -rn -b1
  6254 bytes (6.1k)
local copy
pigz -11 -n
  6247 bytes (6.1k)
local copy
Zopfli
  6240 bytes (6.1k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.9.2.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 1.9.2 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 1049 bytes by using my UnderscoreJS 1.9.2 Zopfli version instead of the best available CDN (16.81% smaller than unpkg, 6240 vs. 7289 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 --mls4096 --bsr18 --lazy --ohh

(found January 9, 2020)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 4096  --mls4096
block splitting recursion 18  --bsr18
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/1.9.2/underscore-min.js --location | md5sum
b87f566fe06d9943ad7fe234667a8154  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.9.2.min.zopfli.js.gz | md5sum
b87f566fe06d9943ad7fe234667a8154  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.9.2/underscore-min.js --location | sha1sum
c08fb4e7f1a3c351f2fbc1b6986e6a487628b54a  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.9.2.min.zopfli.js.gz | sha1sum
c08fb4e7f1a3c351f2fbc1b6986e6a487628b54a  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
unpkg 7289 bytes b87f566fe06d9943ad7fe234667a8154 (invalid)

Note: only the MD5 hashes are shown to keep things simple.

Other Versions

Available UnderscoreJS versions at minime.stephan-brumme.com:

1.9.2, 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
6240 bytes -1 byte zopfli --i10000 --mls4096 --bsr18 --lazy --ohh January 9, 2020 @ 14:31
6241 bytes -1 byte zopfli --i10000 --mls512 --bsr12 --lazy --ohh January 9, 2020 @ 14:30
6242 bytes zopfli --i100 --mls512 --bsr14 --lazy --ohh January 9, 2020 @ 14:21

If there are multiple parameter sets yielding the same compressed size, only the first one found is shown.

Most recent activity on January 9, 2020 @ 19:05.

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, 100,000 or 1,000,000.
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
6251 6251 6251 6251 6252 6252 6251 6250 6251 6251 6251 6243 6253 6252 6252
6255 6255 6260 6260 6260 6254 6245 6258 6246 6242 6245 6249 6247 6245 6245
6258 6258 6258 6256 6258 6254 6244 6245 6245 6248 6245 6245 6246 6248 6249
6254 6254 6258 6259 6257 6245 6257 6250 6245 6245 6244 6244 6247 6246 6245
6253 6253 6255 6255 6252 6251 6244 6242 6251 6244 6245 6243 6245 6248 6248
6256 6256 6258 6244 6255 6243 6246 6244 6240 6244 6244 6240 6247 6245 6245
6258 6258 6257 6257 6257 6258 6258 6243 6244 6244 6244 6245 6247 6245 6245
6252 6252 6254 6246 6243 6244 6244 6250 6246 6240 6244 6245 6245 6247 6247
6259 6259 6257 6260 6257 6248 6257 6245 6240 6245 6245 6249 6245 6251 6251
6259 6259 6257 6257 6257 6257 6256 6242 6243 6245 6245 6245 6246 6247 6248
6255 6255 6258 6242 6253 6255 6253 6242 6241 6241 6245 6245 6245 6247 6248
6254 6254 6259 6257 6257 6245 6257 6249 6245 6245 6245 6242 6246 6245 6245
6259 6259 6257 6257 6257 6256 6246 6257 6243 6240 6244 6245 6244 6248 6247
6259 6259 6257 6257 6258 6251 6256 6250 6243 6240 6245 6246 6244 6247 6251
6259 6259 6257 6261 6257 6254 6257 6242 6241 6245 6245 6240 6247 6245 6246
6253 6253 6255 6241 6257 6244 6246 6250 6246 6245 6244 6246 6246 6245 6247
6259 6259 6255 6260 6257 6256 6257 6258 6243 6248 6244 6244 6245 6245 6245
6258 6258 6257 6257 6257 6255 6258 6244 6240 6244 6244 6245 6245 6245 6246
6259 6259 6255 6253 6255 6254 6245 6242 6241 6248 6244 6242 6246 6245 6246
6259 6259 6257 6260 6258 6245 6257 6257 6245 6244 6244 6244 6244 6248 6245
6259 6259 6256 6260 6257 6244 6256 6244 6244 6244 6244 6244 6240 6245 6245
6259 6259 6257 6254 6257 6254 6257 6243 6243 6240 6244 6245 6246 6245 6247
6259 6259 6257 6258 6257 6253 6256 6253 6244 6248 6244 6240 6246 6247 6248

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 6242 bytes 100%
1,000 6242 bytes 100%
10,000 6240 bytes -2 bytes 100%
100,000 6240 bytes 4.64%
1,000,000 6240 bytes 3.19%
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
6254 bytes +14 bytes (+0.22%)
6254 bytes +14 bytes (+0.22%)
6274 bytes +34 bytes (+0.54%) +20 bytes
6275 bytes +35 bytes (+0.56%) +21 bytes
6301 bytes +61 bytes (+0.98%) +47 bytes
6335 bytes +95 bytes (+1.52%) +81 bytes
6370 bytes +130 bytes (+2.08%) +116 bytes
6406 bytes +166 bytes (+2.66%) +152 bytes
6440 bytes +200 bytes (+3.21%) +186 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 - for example, your browser actually supports it !
Algorithm Program Parameters Size Compared To Best Zopfli
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 5493 bytes -747 bytes (-11.97%)
Brotli (Wikipedia) brotli brotli -q 11 5841 bytes -399 bytes (-6.39%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 5855 bytes -385 bytes (-6.17%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 6133 bytes -107 bytes (-1.71%)
Zstandard (Wikipedia) zstd zstd -19 6237 bytes -3 bytes (-0.05%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 6239 bytes -1 byte (-0.02%)
LZMA2 (Wikipedia) xz xz -9 6280 bytes +40 bytes (+0.64%)

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 2020.
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

All trademarks are property of their respective owners. You know, the boring legal stuff.