Choose a version:
35% The original file has 34705 bytes (33.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 12019 bytes (11.7k, 35%).

After GZIP compression these minified files vary in size:
Boot
  4552 bytes (4.4k)
CDN
Baidu
  4090 bytes (4.0k)
CDN
gzip -6 (default)
  4076 bytes (4.0k)
local copy
gzip -9
  4069 bytes (4.0k)
local copy
cdnjs
  4067 bytes (4.0k)
CDN
cdnhttps
  4052 bytes (4.0k)
CDN
unpkg
  4052 bytes (4.0k)
CDN
libdeflate -12
  3967 bytes (3.9k)
local copy
7zip -mx=9 -tgzip
  3962 bytes (3.9k)
local copy
kzip -s0 -rn -b0
  3961 bytes (3.9k)
local copy
pigz -11 -n
  3954 bytes (3.9k)
local copy
Zopfli
  3945 bytes (3.9k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.2.3.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 1.2.3 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 107 bytes by using my UnderscoreJS 1.2.3 Zopfli version instead of the best available CDN (2.71% smaller than unpkg, 3945 vs. 4052 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 --mls128 --bsr8 --lazy --ohh

(found September 17, 2015)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 128  --mls128
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/1.2.3/underscore-min.js --location | md5sum
eb40cdf14de93e997357c9c30017b2e6  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.2.3.min.zopfli.js.gz | md5sum
eb40cdf14de93e997357c9c30017b2e6  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.2.3/underscore-min.js --location | sha1sum
cfbc3fa20d685d100308e123c3310dc9600ef0fd  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.2.3.min.zopfli.js.gz | sha1sum
cfbc3fa20d685d100308e123c3310dc9600ef0fd  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 4552 bytes eb40cdf14de93e997357c9c30017b2e6 (invalid)
cdnjs 4067 bytes eb40cdf14de93e997357c9c30017b2e6 (invalid)
cdnhttps 4052 bytes eb40cdf14de93e997357c9c30017b2e6 December 24, 2015 @ 07:36
unpkg 4052 bytes eb40cdf14de93e997357c9c30017b2e6 July 11, 2016 @ 16:49

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 4090 bytes 6a94c3bb265771bf56c49a692ef6f728 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:

1.9.1, 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
3945 bytes -1 byte zopfli --i100000 --mls128 --bsr8 --lazy --ohh September 17, 2015 @ 17:03
3946 bytes -2 bytes zopfli --i1000 --mls4096 --bsr6 --lazy --ohh September 8, 2015 @ 10:58
3948 bytes zopfli --i100 --mls4096 --bsr6 --lazy --ohh September 8, 2015 @ 10:35

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 or 100,000.
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
3951 3951 3951 3950 3951 3951 3950 3950 3950 3951 3951 3950 3949 3949 3949
3950 3950 3950 3949 3949 3949 3953 3945 3948 3950 3953 3949 3948 3949 3949
3949 3949 3949 3949 3949 3950 3948 3949 3948 3947 3950 3945 3947 3947 3947
3949 3949 3949 3945 3952 3949 3949 3945 3948 3950 3947 3948 3947 3950 3950
3950 3950 3949 3948 3949 3949 3945 3945 3949 3950 3948 3948 3947 3950 3950
3949 3949 3949 3948 3949 3949 3946 3945 3948 3950 3947 3947 3947 3947 3947
3949 3949 3950 3949 3950 3949 3949 3945 3948 3951 3947 3947 3947 3950 3950
3949 3949 3949 3949 3949 3949 3953 3945 3954 3950 3953 3947 3945 3945 3945
3949 3949 3949 3948 3950 3949 3945 3945 3948 3950 3947 3948 3948 3947 3947
3949 3949 3950 3949 3949 3949 3949 3948 3949 3950 3947 3947 3947 3947 3947
3949 3949 3949 3948 3949 3949 3953 3953 3948 3950 3953 3948 3947 3947 3947
3949 3949 3949 3949 3949 3949 3947 3947 3948 3950 3947 3947 3947 3950 3950
3953 3953 3953 3948 3949 3949 3947 3945 3949 3950 3948 3947 3947 3947 3947
3949 3949 3949 3949 3949 3949 3947 3946 3948 3950 3947 3947 3947 3947 3947
3949 3949 3949 3949 3950 3950 3947 3945 3947 3950 3947 3947 3947 3947 3947
3949 3949 3949 3949 3949 3951 3953 3945 3948 3950 3947 3947 3947 3947 3947
3949 3949 3949 3949 3950 3949 3947 3945 3947 3948 3947 3947 3947 3947 3947
3949 3949 3949 3949 3949 3950 3947 3945 3949 3950 3947 3947 3947 3947 3947
3949 3949 3949 3949 3950 3950 3949 3945 3948 3951 3947 3948 3948 3949 3949
3949 3949 3949 3949 3949 3949 3947 3945 3948 3950 3948 3948 3947 3947 3947
3949 3949 3950 3949 3950 3953 3949 3945 3950 3951 3949 3948 3947 3947 3947
3949 3949 3949 3949 3949 3949 3945 3945 3948 3947 3947 3948 3947 3947 3947
3949 3949 3949 3948 3949 3949 3949 3947 3949 3950 3947 3947 3948 3949 3949

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 3948 bytes 100%
1,000 3946 bytes -2 bytes 100%
10,000 3946 bytes 100%
100,000 3945 bytes -1 byte 16.23%
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
3961 bytes +16 bytes (+0.41%)
3961 bytes +16 bytes (+0.41%)
3981 bytes +36 bytes (+0.91%) +20 bytes
3979 bytes +34 bytes (+0.86%) +18 bytes
4010 bytes +65 bytes (+1.65%) +49 bytes
4033 bytes +88 bytes (+2.23%) +72 bytes
4065 bytes +120 bytes (+3.04%) +104 bytes
4091 bytes +146 bytes (+3.70%) +130 bytes
4124 bytes +179 bytes (+4.54%) +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 - 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 3539 bytes -406 bytes (-10.29%)
Brotli (Wikipedia) brotli brotli -q 11 3630 bytes -315 bytes (-7.98%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 3902 bytes -43 bytes (-1.09%)
Zstandard (Wikipedia) zstd zstd -19 3978 bytes +33 bytes (+0.84%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 4005 bytes +60 bytes (+1.52%)
LZMA2 (Wikipedia) xz xz -9 4036 bytes +91 bytes (+2.31%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 4398 bytes +453 bytes (+11.48%)

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