Choose a version:
29% The original file has 68420 bytes (66.8k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 19530 bytes (19.1k, 29%).

After GZIP compression these minified files vary in size:
unpkg
  8938 bytes (8.7k)
CDN
gzip -6 (default)
  7517 bytes (7.3k)
local copy
gzip -9
  7509 bytes (7.3k)
local copy
libdeflate -12
  7326 bytes (7.2k)
local copy
7zip -mx=9 -tgzip
  7315 bytes (7.1k)
local copy
kzip -s0 -rn -b2
  7313 bytes (7.1k)
local copy
zultra
  7312 bytes (7.1k)
local copy
pigz -11 -n
  7305 bytes (7.1k)
local copy
Zopfli
  7295 bytes (7.1k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.13.0.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 1.13.0 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 1643 bytes by using my UnderscoreJS 1.13.0 Zopfli version instead of the best available CDN (22.52% smaller than unpkg, 7295 vs. 8938 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 --i100000 --mb8 --mls2048 --bsr30 --lazy --ohh

(found July 22, 2021)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 2048  --mls2048
block splitting recursion 30  --bsr30
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/1.13.0/underscore-min.js --location | md5sum
34c9b64e8c5bdf1a1b6e58e743cb4e8d  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.13.0.min.zopfli.js.gz | md5sum
34c9b64e8c5bdf1a1b6e58e743cb4e8d  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.13.0/underscore-min.js --location | sha1sum
9bf3c20f28d40fe9ad0fd0be9d435174b355b918  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.13.0.min.zopfli.js.gz | sha1sum
9bf3c20f28d40fe9ad0fd0be9d435174b355b918  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
unpkg 8938 bytes 34c9b64e8c5bdf1a1b6e58e743cb4e8d (invalid)

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

Other Versions

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

1.13.1, 1.13.0,
1.12.1, 1.12.0,
1.11.0,
1.10.2, 1.10.1, 1.10.0,
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
7295 bytes -1 byte zopfli --i100000 --mls2048 --bsr30 --lazy --ohh July 22, 2021 @ 10:54
7296 bytes -1 byte zopfli --i10000 --mls16384 --bsr25 --lazy --ohh July 21, 2021 @ 16:27
7297 bytes -1 byte zopfli --i1000 --mls2048 --bsr15 --lazy --ohh July 21, 2021 @ 14:34
7298 bytes -1 byte zopfli --i1000 --mls2048 --bsr13 --lazy --ohh July 21, 2021 @ 14:34
7299 bytes -1 byte zopfli --i1000 --mls32768 --bsr9 --lazy --ohh July 21, 2021 @ 14:22
7300 bytes zopfli --i100 --mls16384 --bsr25 --lazy --ohh July 21, 2021 @ 14:08

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

Most recent activity on July 22, 2021 @ 16:24.

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
7307 7307 7307 7307 7307 7300 7306 7309 7307 7298 7299 7300 7296 7296 7297
7305 7305 7302 7306 7302 7305 7303 7306 7307 7303 7299 7298 7298 7298 7298
7306 7305 7305 7302 7304 7305 7305 7306 7303 7303 7297 7299 7298 7296 7298
7306 7302 7304 7302 7302 7302 7302 7300 7306 7300 7299 7298 7298 7298 7298
7302 7302 7305 7302 7304 7298 7302 7302 7304 7301 7300 7298 7298 7297 7297
7302 7301 7302 7302 7301 7305 7302 7302 7298 7299 7295 7297 7298 7303 7298
7311 7305 7312 7305 7304 7298 7298 7299 7301 7301 7299 7298 7298 7296 7296
7304 7302 7305 7302 7305 7298 7304 7302 7304 7301 7299 7298 7298 7297 7296
7305 7301 7305 7301 7302 7302 7302 7303 7307 7305 7303 7298 7298 7296 7297
7304 7305 7306 7302 7304 7302 7306 7306 7304 7298 7295 7302 7298 7297 7296
7306 7305 7305 7302 7304 7298 7305 7302 7306 7301 7299 7298 7298 7296 7299
7302 7302 7302 7302 7304 7302 7301 7302 7304 7301 7296 7298 7298 7298 7297
7306 7305 7304 7305 7305 7302 7304 7305 7298 7306 7299 7298 7298 7298 7298
7305 7302 7305 7307 7305 7298 7305 7306 7307 7305 7299 7297 7301 7296 7298
7305 7302 7302 7302 7302 7302 7305 7303 7304 7301 7299 7303 7298 7296 7298
7306 7305 7304 7302 7302 7302 7305 7302 7298 7299 7297 7298 7302 7299 7298
7302 7305 7305 7302 7302 7302 7305 7302 7304 7304 7299 7302 7298 7297 7296
7306 7305 7302 7303 7305 7305 7302 7302 7301 7305 7299 7298 7298 7298 7298
7305 7301 7305 7302 7304 7302 7305 7302 7298 7299 7299 7297 7302 7296 7299
7302 7301 7305 7303 7302 7298 7305 7302 7304 7301 7299 7298 7298 7298 7298
7306 7301 7302 7302 7304 7303 7305 7302 7301 7298 7299 7302 7298 7296 7297
7302 7303 7302 7302 7305 7302 7302 7302 7303 7301 7295 7297 7298 7296 7297
7302 7302 7304 7301 7302 7302 7302 7303 7298 7302 7299 7302 7298 7297 7297

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 7300 bytes 100%
1,000 7297 bytes -3 bytes 100%
10,000 7296 bytes -1 byte 100%
100,000 7295 bytes -1 byte 5.51%
1,000,000 7295 bytes 0.87%
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
7314 bytes +19 bytes (+0.26%) +1 byte
7314 bytes +19 bytes (+0.26%) +1 byte
7313 bytes +18 bytes (+0.25%)
7336 bytes +41 bytes (+0.56%) +23 bytes
7363 bytes +68 bytes (+0.93%) +50 bytes
7390 bytes +95 bytes (+1.30%) +77 bytes
7429 bytes +134 bytes (+1.84%) +116 bytes
7454 bytes +159 bytes (+2.18%) +141 bytes
7474 bytes +179 bytes (+2.45%) +161 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 6566 bytes -729 bytes (-9.99%)
Brotli (Wikipedia) brotli brotli -q 11 6842 bytes -453 bytes (-6.21%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 6966 bytes -329 bytes (-4.51%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 7260 bytes -35 bytes (-0.48%)
LZMA2 (Wikipedia) xz xz -9 7272 bytes -23 bytes (-0.32%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 7292 bytes -3 bytes (-0.04%)
Zstandard (Wikipedia) zstd zstd -19 7292 bytes -3 bytes (-0.04%)

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 2021.
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.