Choose a version:
33% The original file has 42933 bytes (41.9k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 14241 bytes (13.9k, 33%).

After GZIP compression these minified files vary in size:
Boot
  5494 bytes (5.4k)
CDN
gzip -6 (default)
  4947 bytes (4.8k)
local copy
Baidu
  4940 bytes (4.8k)
CDN
cdnjs
  4938 bytes (4.8k)
CDN
gzip -9
  4933 bytes (4.8k)
local copy
cdnhttps
  4923 bytes (4.8k)
CDN
7zip -mx=9 -tgzip
  4812 bytes (4.7k)
local copy
libdeflate -12
  4809 bytes (4.7k)
local copy
kzip -s0 -rn -b1
  4806 bytes (4.7k)
local copy
pigz -11 -n
  4803 bytes (4.7k)
local copy
Zopfli
  4797 bytes (4.7k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.5.0.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 1.5.0 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 126 bytes by using my UnderscoreJS 1.5.0 Zopfli version instead of the best available CDN (2.63% smaller than cdnhttps, 4797 vs. 4923 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 --i10000 --mb8 --mls2048 --bsr4 --lazy --ohh

(found September 8, 2015)
Description Value Parameter
iterations 10000  --i10000
maximum blocks 8  --mb8
maximum length score 2048  --mls2048
block splitting recursion 4  --bsr4
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/1.5.0/underscore-min.js --location | md5sum
cc07a4658799e1512b086467e7ef5ca5  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.5.0.min.zopfli.js.gz | md5sum
cc07a4658799e1512b086467e7ef5ca5  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.5.0/underscore-min.js --location | sha1sum
44a4cae62496c4f713ae3a6f1d2b32cf42933069  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.5.0.min.zopfli.js.gz | sha1sum
44a4cae62496c4f713ae3a6f1d2b32cf42933069  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 5494 bytes cc07a4658799e1512b086467e7ef5ca5 (invalid)
cdnjs 4938 bytes cc07a4658799e1512b086467e7ef5ca5 (invalid)
cdnhttps 4923 bytes cc07a4658799e1512b086467e7ef5ca5 December 24, 2015 @ 07:36

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 4940 bytes 7c9deae1501c50f661db5702d361a122 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
4797 bytes -1 byte zopfli --i10000 --mls2048 --bsr4 --lazy --ohh September 8, 2015 @ 13:14
4798 bytes -1 byte zopfli --i1000 --mls2048 --bsr9 --lazy --ohh September 8, 2015 @ 10:47
4799 bytes zopfli --i100 --mls2048 --bsr4 --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 June 16, 2016 @ 14:39.

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
4801 4801 4801 4801 4801 4801 4801 4799 4801 4797 4798 4800 4801 4802 4802
4801 4801 4802 4799 4799 4800 4800 4797 4798 4798 4797 4801 4800 4800 4800
4800 4800 4799 4801 4797 4797 4800 4798 4798 4798 4799 4801 4801 4800 4800
4799 4799 4799 4801 4800 4801 4800 4801 4801 4797 4799 4797 4797 4798 4798
4801 4801 4799 4801 4799 4800 4802 4802 4801 4798 4798 4800 4800 4800 4800
4803 4803 4801 4800 4799 4800 4800 4798 4801 4798 4797 4801 4800 4798 4798
4800 4800 4799 4802 4801 4801 4801 4801 4798 4798 4797 4801 4797 4800 4800
4800 4800 4800 4800 4799 4801 4802 4801 4799 4798 4798 4800 4808 4800 4800
4799 4799 4800 4801 4799 4798 4801 4799 4800 4798 4799 4797 4800 4800 4800
4799 4799 4800 4801 4801 4800 4800 4801 4801 4800 4797 4801 4801 4800 4800
4800 4800 4800 4801 4799 4800 4802 4798 4800 4798 4798 4799 4800 4799 4799
4800 4800 4799 4799 4800 4803 4802 4801 4798 4797 4799 4799 4799 4798 4798
4800 4800 4800 4799 4799 4803 4800 4801 4798 4798 4798 4801 4801 4800 4800
4801 4801 4799 4800 4801 4797 4800 4798 4799 4798 4798 4800 4800 4801 4801
4800 4800 4802 4799 4799 4801 4800 4801 4802 4797 4799 4799 4798 4798 4798
4800 4800 4799 4800 4799 4799 4801 4797 4798 4798 4797 4801 4797 4798 4798
4799 4799 4799 4807 4801 4800 4802 4799 4799 4798 4797 4800 4799 4800 4800
4800 4800 4800 4801 4801 4801 4800 4799 4800 4797 4798 4801 4800 4800 4800
4800 4800 4800 4800 4799 4801 4802 4801 4798 4798 4797 4801 4800 4798 4798
4800 4800 4802 4800 4800 4803 4801 4797 4798 4798 4797 4801 4800 4798 4798
4800 4800 4799 4801 4801 4803 4800 4801 4799 4800 4799 4797 4800 4800 4800
4800 4800 4799 4800 4799 4799 4802 4801 4800 4800 4798 4801 4800 4800 4800
4800 4800 4799 4801 4798 4801 4800 4798 4799 4798 4797 4799 4800 4800 4800

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 4799 bytes 100%
1,000 4798 bytes -1 byte 100%
10,000 4797 bytes -1 byte 100%
100,000 4797 bytes 3.19%
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
4806 bytes +9 bytes (+0.19%)
4806 bytes +9 bytes (+0.19%)
4825 bytes +28 bytes (+0.58%) +19 bytes
4826 bytes +29 bytes (+0.60%) +20 bytes
4857 bytes +60 bytes (+1.25%) +51 bytes
4891 bytes +94 bytes (+1.96%) +85 bytes
4929 bytes +132 bytes (+2.75%) +123 bytes
4959 bytes +162 bytes (+3.38%) +153 bytes
4986 bytes +189 bytes (+3.94%) +180 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 4262 bytes -535 bytes (-11.15%)
Brotli (Wikipedia) brotli brotli -q 11 4465 bytes -332 bytes (-6.92%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 4790 bytes -7 bytes (-0.15%)
Zstandard (Wikipedia) zstd zstd -19 4807 bytes +10 bytes (+0.21%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 4837 bytes +40 bytes (+0.83%)
LZMA2 (Wikipedia) xz xz -9 4864 bytes +67 bytes (+1.40%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 5113 bytes +316 bytes (+6.59%)

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