Choose a version:
34% The original file has 33068 bytes (32.3k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 11383 bytes (11.1k, 34%).

After GZIP compression these minified files vary in size:
Boot
  4360 bytes (4.3k)
CDN
Baidu
  3923 bytes (3.8k)
CDN
gzip -6 (default)
  3909 bytes (3.8k)
local copy
gzip -9
  3909 bytes (3.8k)
local copy
unpkg
  3906 bytes (3.8k)
CDN
cdnjs
  3903 bytes (3.8k)
CDN
cdnhttps
  3885 bytes (3.8k)
CDN
7zip -mx=9 -tgzip
  3821 bytes (3.7k)
local copy
libdeflate -12
  3820 bytes (3.7k)
local copy
kzip -s0 -rn -b0
  3812 bytes (3.7k)
local copy
pigz -11 -n
  3806 bytes (3.7k)
local copy
Zopfli
  3801 bytes (3.7k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.2.0.min.js

You will automatically get the smallest UnderscoreJS 1.2.0 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 84 bytes by using my UnderscoreJS 1.2.0 Zopfli version instead of the best available CDN (2.21% smaller than cdnhttps, 3801 vs. 3885 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 --mls8192 --bsr11 --lazy --ohh

(found September 8, 2015)
Description Value Parameter
iterations 1000  --i1000
maximum blocks 8  --mb8
maximum length score 8192  --mls8192
block splitting recursion 11  --bsr11
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/1.2.0/underscore-min.js --location | md5sum
e60157694a7ce538626c645dfd910533  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.2.0.min.zopfli.js.gz | md5sum
e60157694a7ce538626c645dfd910533  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.2.0/underscore-min.js --location | sha1sum
7d6e352b0d26655ba851863561b5d912cc24caa7  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.2.0.min.zopfli.js.gz | sha1sum
7d6e352b0d26655ba851863561b5d912cc24caa7  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 4360 bytes e60157694a7ce538626c645dfd910533 (invalid)
unpkg 3906 bytes e60157694a7ce538626c645dfd910533 July 11, 2016 @ 16:49
cdnjs 3903 bytes e60157694a7ce538626c645dfd910533 (invalid)
cdnhttps 3885 bytes e60157694a7ce538626c645dfd910533 December 24, 2015 @ 07:36

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 3923 bytes f13031440fe4a2fc36d57a76d9fb395c 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
3801 bytes -1 byte zopfli --i1000 --mls8192 --bsr11 --lazy --ohh September 8, 2015 @ 11:01
3802 bytes zopfli --i100 --mls1024 --bsr11 --lazy --ohh September 8, 2015 @ 10:34

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
3808 3808 3807 3808 3810 3810 3809 3809 3810 3810 3808 3808 3806 3806 3806
3802 3802 3802 3802 3803 3801 3802 3802 3802 3802 3804 3802 3802 3803 3803
3804 3804 3804 3805 3802 3802 3802 3802 3802 3802 3802 3803 3801 3801 3801
3806 3806 3803 3805 3801 3802 3803 3802 3802 3802 3802 3803 3801 3802 3802
3805 3805 3801 3803 3801 3802 3802 3804 3802 3801 3804 3802 3801 3801 3801
3802 3802 3805 3803 3802 3802 3802 3802 3802 3802 3801 3803 3801 3802 3802
3804 3804 3802 3803 3803 3802 3802 3805 3801 3802 3801 3801 3801 3802 3802
3807 3807 3801 3802 3801 3805 3809 3802 3801 3801 3804 3801 3801 3802 3802
3805 3805 3802 3802 3801 3811 3809 3809 3808 3802 3808 3801 3802 3802 3802
3801 3801 3802 3803 3803 3802 3802 3802 3802 3802 3802 3802 3801 3802 3802
3809 3809 3804 3804 3801 3802 3802 3802 3802 3802 3801 3802 3801 3801 3801
3807 3807 3802 3804 3801 3806 3809 3804 3809 3802 3804 3803 3801 3801 3801
3801 3801 3802 3802 3802 3802 3803 3802 3801 3801 3801 3802 3801 3802 3802
3805 3805 3801 3805 3801 3802 3802 3802 3802 3801 3801 3801 3802 3801 3801
3805 3805 3801 3802 3801 3802 3802 3802 3801 3805 3804 3801 3801 3801 3801
3801 3801 3803 3802 3801 3802 3802 3802 3801 3801 3802 3801 3801 3801 3801
3801 3801 3803 3804 3801 3802 3802 3802 3801 3801 3801 3804 3801 3801 3801
3801 3801 3801 3802 3802 3802 3802 3802 3801 3801 3804 3801 3801 3803 3803
3801 3801 3803 3802 3803 3808 3809 3802 3802 3802 3803 3803 3801 3802 3802
3805 3805 3804 3805 3803 3802 3802 3802 3801 3802 3804 3802 3801 3802 3802
3804 3804 3802 3802 3802 3802 3802 3802 3802 3801 3804 3802 3802 3802 3802
3803 3803 3801 3803 3802 3802 3802 3802 3801 3802 3802 3802 3801 3802 3802
3801 3801 3802 3802 3802 3802 3802 3802 3802 3802 3804 3801 3802 3803 3803

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 3802 bytes 100%
1,000 3801 bytes -1 byte 100%
10,000 3801 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
3812 bytes +11 bytes (+0.29%)
3812 bytes +11 bytes (+0.29%)
3835 bytes +34 bytes (+0.89%) +23 bytes
3839 bytes +38 bytes (+1.00%) +27 bytes
3858 bytes +57 bytes (+1.50%) +46 bytes
3890 bytes +89 bytes (+2.34%) +78 bytes
3919 bytes +118 bytes (+3.10%) +107 bytes
3954 bytes +153 bytes (+4.03%) +142 bytes
3985 bytes +184 bytes (+4.84%) +173 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 3426 bytes -375 bytes (-9.87%)
Brotli (Wikipedia) brotli brotli -q 11 3494 bytes -307 bytes (-8.08%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 3803 bytes +2 bytes (+0.05%)
ZSTD (Wikipedia) zstd zstd -19 3829 bytes +28 bytes (+0.74%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 3873 bytes +72 bytes (+1.89%)
LZMA2 (Wikipedia) xz xz -9 3896 bytes +95 bytes (+2.50%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 4285 bytes +484 bytes (+12.73%)

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