Choose a version:
34% The original file has 37390 bytes (36.5k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 12821 bytes (12.5k, 34%).

After GZIP compression these minified files vary in size:
Boot
  4904 bytes (4.8k)
 CDN
Baidu
  4418 bytes (4.3k)
 CDN
gzip -6 (default)
  4410 bytes (4.3k)
 local copy
gzip -9
  4401 bytes (4.3k)
 local copy
cdnjs
  4396 bytes (4.3k)
 CDN
unpkg
  4393 bytes (4.3k)
 CDN
libdeflate -12
  4282 bytes (4.2k)
 local copy
7zip -mx=9 -tgzip
  4280 bytes (4.2k)
 local copy
kzip -s0 -rn -b0
  4277 bytes (4.2k)
 local copy
zultra
  4277 bytes (4.2k)
 local copy
pigz -11 -n
  4266 bytes (4.2k)
 local copy
Zopfli
  4259 bytes (4.2k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.3.3.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 1.3.3 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 134 bytes by using my UnderscoreJS 1.3.3 Zopfli version instead of the best available CDN (3.15% smaller than unpkg, 4259 vs. 4393 bytes):
Download
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 --i1000000 --mb8 --mls1024 --bsr8 --lazy --ohh

(found February 26, 2020)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 1024  --mls1024
block splitting recursion 8  --bsr8
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/1.3.3/underscore-min.js --location | md5sum
0899f764c85bbcc13206e3dd8318e678  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.3.3.min.zopfli.js.gz | md5sum
0899f764c85bbcc13206e3dd8318e678  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.3.3/underscore-min.js --location | sha1sum
baa693e82387a328abccf9694c6d2db571ac8aca  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.3.3.min.zopfli.js.gz | sha1sum
baa693e82387a328abccf9694c6d2db571ac8aca  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 4904 bytes 0899f764c85bbcc13206e3dd8318e678 (invalid)
cdnjs 4396 bytes 0899f764c85bbcc13206e3dd8318e678 (invalid)
unpkg 4393 bytes 0899f764c85bbcc13206e3dd8318e678 July 11, 2016 @ 16:49

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 4418 bytes 13223be203b9c732fbb5766b6ac55dd1 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:

1.13.6, 1.13.5, 1.13.4, 1.13.3, 1.13.2, 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
4259 bytes -1 byte zopfli --i1000000 --mls1024 --bsr8 --lazy --ohh February 26, 2020 @ 19:52
4260 bytes -1 byte zopfli --i10000 --mls1024 --bsr8 --lazy --ohh September 8, 2015 @ 15:07
4261 bytes -1 byte zopfli --i10000 --mls512 --bsr12 --lazy --ohh September 8, 2015 @ 15:01
4262 bytes -1 byte zopfli --i1000 --mls512 --bsr10 --lazy --ohh September 8, 2015 @ 10:53
4263 bytes -1 byte zopfli --i1000 --mls512 --bsr14 --lazy --ohh September 8, 2015 @ 10:53
4264 bytes – zopfli --i100 --mls512 --bsr16 --lazy --ohh September 8, 2015 @ 10:32

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

Most recent activity on July 20, 2020 @ 12:52.

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
2 4269 4269 4277 4268 4267 4277 4277 4265 4269 4268 4268 4267 4269 4268 4268
4 4263 4263 4263 4263 4261 4261 4264 4267 4264 4264 4262 4263 4261 4265 4265
6 4263 4264 4264 4263 4262 4263 4264 4262 4263 4261 4267 4262 4264 4261 4261
7 4275 4275 4275 4275 4261 4263 4264 4261 4261 4264 4261 4262 4261 4261 4261
8 4263 4265 4261 4264 4261 4264 4264 4267 4262 4259 4261 4262 4261 4260 4260
9 4263 4264 4264 4263 4261 4261 4264 4263 4264 4261 4261 4262 4260 4260 4260
10 4263 4264 4264 4263 4263 4263 4260 4264 4261 4263 4261 4264 4261 4261 4261
11 4276 4276 4275 4277 4264 4275 4264 4261 4261 4263 4261 4262 4261 4261 4261
12 4265 4264 4264 4263 4263 4264 4264 4262 4261 4264 4261 4262 4262 4261 4261
13 4264 4264 4262 4263 4264 4263 4264 4267 4261 4264 4262 4262 4260 4261 4261
14 4264 4264 4261 4262 4263 4263 4264 4261 4261 4264 4261 4262 4260 4261 4261
15 4264 4264 4264 4263 4262 4263 4264 4262 4262 4264 4261 4262 4260 4261 4261
16 4263 4265 4264 4263 4261 4263 4264 4264 4261 4264 4262 4262 4260 4260 4260
17 4263 4264 4264 4263 4264 4263 4264 4262 4261 4264 4261 4262 4260 4260 4260
18 4264 4264 4264 4263 4260 4262 4264 4260 4261 4264 4261 4264 4261 4261 4261
19 4264 4264 4264 4264 4263 4263 4264 4261 4263 4261 4261 4262 4261 4261 4261
20 4264 4264 4261 4263 4264 4264 4260 4263 4261 4264 4261 4262 4261 4261 4261
21 4276 4276 4265 4263 4261 4263 4264 4263 4263 4264 4261 4262 4261 4261 4261
22 4263 4264 4261 4263 4263 4263 4264 4263 4261 4264 4261 4262 4261 4261 4261
23 4264 4264 4261 4263 4263 4263 4264 4263 4261 4264 4261 4264 4262 4262 4262
25 4264 4265 4264 4263 4262 4263 4264 4262 4261 4264 4262 4262 4262 4262 4262
30 4265 4265 4264 4264 4262 4264 4264 4264 4261 4261 4261 4262 4261 4262 4262
40 4265 4265 4264 4262 4264 4264 4264 4261 4261 4264 4266 4267 4260 4260 4260

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 4264 bytes – 100%
1,000 4262 bytes -2 bytes 100%
10,000 4260 bytes -2 bytes 100%
100,000 4260 bytes – 6.38%
1,000,000 4259 bytes -1 byte 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
(any) 4277 bytes +18 bytes (+0.42%) –
1 4278 bytes +19 bytes (+0.45%) +1 byte
2 4291 bytes +32 bytes (+0.75%) +14 bytes
3 4290 bytes +31 bytes (+0.73%) +13 bytes
4 4324 bytes +65 bytes (+1.53%) +47 bytes
5 4345 bytes +86 bytes (+2.02%) +68 bytes
6 4376 bytes +117 bytes (+2.75%) +99 bytes
7 4400 bytes +141 bytes (+3.31%) +123 bytes
8 4425 bytes +166 bytes (+3.90%) +148 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 3809 bytes -450 bytes (-10.57%)
Brotli (Wikipedia) brotli brotli -q 11 3937 bytes -322 bytes (-7.56%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 4240 bytes -19 bytes (-0.45%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 4281 bytes +22 bytes (+0.52%)
Zstandard (Wikipedia) zstd zstd -19 4304 bytes +45 bytes (+1.06%)
LZMA2 (Wikipedia) xz xz -9 4352 bytes +93 bytes (+2.18%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 4640 bytes +381 bytes (+8.95%)

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):
Load and Parse Data


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 2024.
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.