Choose a version:
35% The original file has 35168 bytes (34.3k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 12140 bytes (11.9k, 35%).

After GZIP compression these minified files vary in size:
Boot
  4604 bytes (4.5k)
CDN
Baidu
  4130 bytes (4.0k)
CDN
gzip -6 (default)
  4125 bytes (4.0k)
local copy
gzip -9
  4115 bytes (4.0k)
local copy
cdnjs
  4113 bytes (4.0k)
CDN
cdnhttps
  4101 bytes (4.0k)
CDN
unpkg
  4099 bytes (4.0k)
CDN
libdeflate -12
  4012 bytes (3.9k)
local copy
7zip -mx=9 -tgzip
  4008 bytes (3.9k)
local copy
kzip -s0 -rn -b1
  4006 bytes (3.9k)
local copy
pigz -11 -n
  3996 bytes (3.9k)
local copy
Zopfli
  3991 bytes (3.9k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.3.1.min.js

You will automatically get the smallest UnderscoreJS 1.3.1 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 108 bytes by using my UnderscoreJS 1.3.1 Zopfli version instead of the best available CDN (2.71% smaller than unpkg, 3991 vs. 4099 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 --i1000 --mb8 --mls2048 --bsr9 --lazy --ohh

(found September 8, 2015)
Description Value Parameter
iterations 1000  --i1000
maximum blocks 8  --mb8
maximum length score 2048  --mls2048
block splitting recursion 9  --bsr9
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/1.3.1/underscore-min.js --location | md5sum
b538b8a3ed2c5519b19409eace25c38b  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.3.1.min.zopfli.js.gz | md5sum
b538b8a3ed2c5519b19409eace25c38b  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.3.1/underscore-min.js --location | sha1sum
0a15952061f6fb7a5493281f6ff8eb916a417e2f  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.3.1.min.zopfli.js.gz | sha1sum
0a15952061f6fb7a5493281f6ff8eb916a417e2f  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 4604 bytes b538b8a3ed2c5519b19409eace25c38b (invalid)
cdnjs 4113 bytes b538b8a3ed2c5519b19409eace25c38b (invalid)
cdnhttps 4101 bytes b538b8a3ed2c5519b19409eace25c38b December 24, 2015 @ 07:36
unpkg 4099 bytes b538b8a3ed2c5519b19409eace25c38b July 11, 2016 @ 16:49

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 4130 bytes 5831dba73eb2a76d1b662b244f2c2844 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
3991 bytes -1 byte zopfli --i1000 --mls2048 --bsr10 --lazy --ohh September 8, 2015 @ 10:55
3992 bytes -2 bytes zopfli --i1000 --mls512 --bsr4 --lazy --ohh September 8, 2015 @ 10:55
3994 bytes zopfli --i100 --mls4096 --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 11, 2016 @ 16:49.

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.
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
3995 3995 3995 3998 3998 3998 3997 3997 3997 3997 3998 3996 3993 3994 3994
3991 3991 3994 3994 3992 3994 3993 3991 3991 3991 3993 3991 3993 3992 3992
3993 3993 3991 3993 3993 3993 3993 3993 3992 3997 3991 3991 3993 3993 3993
3991 3991 3991 3991 3993 3993 3992 3991 3992 3993 3993 3993 3993 3993 3993
3994 3994 3993 3993 3993 3992 3994 3991 3993 3991 3994 3991 3993 3993 3993
3991 3991 3991 3991 3991 3992 3992 3994 3993 3992 3991 3993 3993 3995 3995
3991 3991 3993 3993 3993 3992 3992 3991 3993 3991 3991 3994 3991 3991 3991
3991 3991 3994 3993 3993 3994 3994 3991 3992 3992 3991 3991 3993 3994 3994
3992 3992 3991 3993 3993 3993 3993 3992 3993 3995 3991 3992 3991 3993 3993
3991 3991 3993 3991 3992 3994 3994 3991 3992 3992 3991 3991 3993 3993 3993
3991 3991 3994 3996 3993 3994 3994 3991 3993 3992 3993 3993 3994 3993 3993
3991 3991 3991 3991 3993 3994 3993 3991 3993 3993 3993 3993 3993 3993 3993
3991 3991 3991 3992 3993 3992 3993 3991 3993 3993 3991 3991 3993 3993 3993
3991 3991 3991 3991 3997 3992 3999 3991 3995 3993 3991 3993 3994 3991 3991
3991 3991 3993 3991 3991 3993 3992 3992 3993 3991 3991 3994 3993 3994 3994
3993 3993 3991 3991 3993 3992 3992 3992 3993 3991 3991 3993 3991 3991 3991
3993 3993 3991 3993 3993 3993 3992 3991 3993 3991 3991 3991 3994 3994 3994
3993 3993 3994 3991 3993 3992 3992 3992 3993 3992 3991 3991 3993 3993 3993
3991 3991 3991 3997 3993 3993 3992 3991 3993 3993 3991 3991 3991 3994 3994
3991 3991 3993 3993 3998 3992 3992 3992 3993 3993 3991 3992 3994 3994 3994
3991 3991 3992 3993 3993 3993 3994 3991 3993 3991 3993 3992 3991 3994 3994
3991 3991 3991 3996 3993 3992 3994 3991 3993 3991 3991 3993 3993 3993 3993
3991 3991 3991 3993 3993 3992 3993 3991 3993 3996 3991 3993 3994 3991 3991

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 3993 bytes 100%
1,000 3991 bytes -2 bytes 100%
10,000 3991 bytes 100%
100,000
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
4006 bytes +15 bytes (+0.38%)
4006 bytes +15 bytes (+0.38%)
4024 bytes +33 bytes (+0.83%) +18 bytes
4022 bytes +31 bytes (+0.78%) +16 bytes
4042 bytes +51 bytes (+1.28%) +36 bytes
4075 bytes +84 bytes (+2.10%) +69 bytes
4108 bytes +117 bytes (+2.93%) +102 bytes
4140 bytes +149 bytes (+3.73%) +134 bytes
4173 bytes +182 bytes (+4.56%) +167 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 3571 bytes -420 bytes (-10.52%)
Brotli (Wikipedia) brotli brotli -q 11 3677 bytes -314 bytes (-7.87%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 3952 bytes -39 bytes (-0.98%)
ZSTD (Wikipedia) zstd zstd -19 4025 bytes +34 bytes (+0.85%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 4037 bytes +46 bytes (+1.15%)
LZMA2 (Wikipedia) xz xz -9 4084 bytes +93 bytes (+2.33%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 4423 bytes +432 bytes (+10.82%)

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