Choose a version:
29% The original file has 68271 bytes (66.7k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 19466 bytes (19.0k, 29%).

After GZIP compression these minified files vary in size:
unpkg
  8266 bytes (8.1k)
CDN
Boot
  7498 bytes (7.3k)
CDN
cdnjs
  7494 bytes (7.3k)
CDN
gzip -6 (default)
  7492 bytes (7.3k)
local copy
gzip -9
  7483 bytes (7.3k)
local copy
libdeflate -12
  7301 bytes (7.1k)
local copy
kzip -s0 -rn -b2
  7289 bytes (7.1k)
local copy
7zip -mx=9 -tgzip
  7287 bytes (7.1k)
local copy
zultra
  7286 bytes (7.1k)
local copy
pigz -11 -n
  7280 bytes (7.1k)
local copy
Zopfli
  7269 bytes (7.1k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.12.1.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 1.12.1 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 225 bytes by using my UnderscoreJS 1.12.1 Zopfli version instead of the best available CDN (3.10% smaller than cdnjs, 7269 vs. 7494 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 --bsr15 --lazy --ohh

(found April 6, 2021)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 2048  --mls2048
block splitting recursion 15  --bsr15
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/1.12.1/underscore-min.js --location | md5sum
f836abbb789375d11ce1092a0f4385c0  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.12.1.min.zopfli.js.gz | md5sum
f836abbb789375d11ce1092a0f4385c0  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.12.1/underscore-min.js --location | sha1sum
6bbbb25f5bcb7c4e5f07a5490107f262f5f261bb  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.12.1.min.zopfli.js.gz | sha1sum
6bbbb25f5bcb7c4e5f07a5490107f262f5f261bb  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 7498 bytes f836abbb789375d11ce1092a0f4385c0 (invalid)
cdnjs 7494 bytes f836abbb789375d11ce1092a0f4385c0 (invalid)

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
unpkg 8266 bytes 9ee425aa01bab40af5934eec1fc71cab < var n="1.12.1",r="object"==typeof self&&self.self===self&& [...]
> var n="1.12.1",r="object"==typeof self&&self.self===self&& [...]
(invalid)

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

Other Versions

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

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
7269 bytes -1 byte zopfli --i100000 --mls2048 --bsr15 --lazy --ohh April 6, 2021 @ 19:41
7270 bytes -1 byte zopfli --i10000 --mls2048 --bsr15 --lazy --ohh April 6, 2021 @ 18:00
7271 bytes -1 byte zopfli --i10000 --mls2048 --bsr9 --lazy --ohh April 6, 2021 @ 17:58
7272 bytes -2 bytes zopfli --i1000 --mls16384 --bsr4 --lazy --ohh April 6, 2021 @ 17:24
7274 bytes zopfli --i100 --mls32768 --bsr21 --lazy --ohh April 6, 2021 @ 16:26

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

Most recent activity on April 7, 2021 @ 10:17.

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
7272 7272 7272 7279 7279 7279 7279 7272 7273 7275 7273 7275 7273 7271 7271
7277 7285 7277 7276 7275 7276 7276 7275 7281 7280 7273 7271 7271 7272 7271
7279 7278 7278 7280 7278 7278 7279 7279 7279 7277 7273 7278 7278 7274 7271
7278 7278 7278 7275 7275 7272 7273 7273 7281 7274 7274 7271 7272 7272 7272
7280 7278 7285 7275 7276 7276 7276 7274 7273 7277 7271 7271 7271 7271 7271
7278 7278 7276 7275 7275 7276 7275 7276 7276 7273 7271 7271 7273 7271 7273
7284 7271 7287 7288 7276 7271 7274 7274 7276 7277 7273 7272 7278 7272 7273
7285 7272 7284 7276 7278 7277 7275 7274 7273 7278 7273 7271 7279 7271 7271
7278 7278 7276 7275 7276 7276 7276 7275 7281 7272 7273 7271 7271 7272 7272
7276 7278 7278 7275 7276 7276 7278 7279 7279 7273 7273 7272 7273 7271 7271
7278 7278 7284 7278 7276 7285 7276 7275 7273 7277 7273 7271 7271 7272 7271
7278 7277 7278 7276 7276 7276 7276 7275 7276 7271 7269 7271 7278 7272 7270
7277 7278 7278 7277 7278 7275 7277 7277 7277 7278 7273 7271 7271 7272 7271
7278 7278 7285 7276 7275 7275 7276 7274 7272 7279 7271 7273 7274 7271 7271
7278 7278 7278 7276 7276 7275 7277 7275 7276 7277 7273 7273 7274 7271 7271
7278 7278 7278 7276 7276 7275 7276 7274 7272 7278 7271 7274 7278 7271 7271
7278 7278 7278 7275 7276 7285 7279 7278 7273 7277 7273 7273 7278 7272 7272
7279 7278 7276 7278 7276 7275 7279 7278 7275 7278 7273 7271 7274 7271 7271
7278 7277 7276 7275 7276 7275 7279 7278 7272 7273 7273 7278 7278 7271 7271
7278 7278 7276 7276 7277 7277 7276 7278 7276 7277 7273 7278 7275 7272 7272
7278 7278 7278 7275 7276 7276 7272 7278 7273 7277 7273 7273 7275 7271 7271
7278 7278 7276 7276 7277 7275 7275 7276 7276 7278 7273 7271 7271 7272 7272
7279 7277 7276 7276 7276 7275 7276 7275 7273 7279 7271 7272 7278 7274 7273

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 7274 bytes 100%
1,000 7272 bytes -2 bytes 100%
10,000 7270 bytes -2 bytes 100%
100,000 7269 bytes -1 byte 0.58%
1,000,000 7269 bytes 0.29%
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
7291 bytes +22 bytes (+0.30%) +2 bytes
7291 bytes +22 bytes (+0.30%) +2 bytes
7289 bytes +20 bytes (+0.28%)
7310 bytes +41 bytes (+0.56%) +21 bytes
7337 bytes +68 bytes (+0.94%) +48 bytes
7365 bytes +96 bytes (+1.32%) +76 bytes
7405 bytes +136 bytes (+1.87%) +116 bytes
7430 bytes +161 bytes (+2.21%) +141 bytes
7452 bytes +183 bytes (+2.52%) +163 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 6541 bytes -728 bytes (-10.02%)
Brotli (Wikipedia) brotli brotli -q 11 6815 bytes -454 bytes (-6.25%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 6941 bytes -328 bytes (-4.51%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 7235 bytes -34 bytes (-0.47%)
LZMA2 (Wikipedia) xz xz -9 7252 bytes -17 bytes (-0.23%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 7259 bytes -10 bytes (-0.14%)
Zstandard (Wikipedia) zstd zstd -19 7263 bytes -6 bytes (-0.08%)

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 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.