Choose a version:
37% The original file has 22692 bytes (22.2k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 8359 bytes (8.2k, 37%).

After GZIP compression these minified files vary in size:
Boot
  3051 bytes (3.0k)
CDN
gzip -6 (default)
  2754 bytes (2.7k)
local copy
gzip -9
  2753 bytes (2.7k)
local copy
Baidu
  2751 bytes (2.7k)
CDN
cdnjs
  2738 bytes (2.7k)
CDN
cdnhttps
  2730 bytes (2.7k)
CDN
7zip -mx=9 -tgzip
  2677 bytes (2.6k)
local copy
libdeflate -12
  2675 bytes (2.6k)
local copy
pigz -11 -n
  2672 bytes (2.6k)
local copy
kzip -s0 -rn -b1
  2671 bytes (2.6k)
local copy
Zopfli
  2667 bytes (2.6k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.7.min.js

You will automatically get the smallest UnderscoreJS 0.5.7 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 63 bytes by using my UnderscoreJS 0.5.7 Zopfli version instead of the best available CDN (2.36% smaller than cdnhttps, 2667 vs. 2730 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 --mls4096 --bsr30 --lazy --ohh

(found February 3, 2016)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 4096  --mls4096
block splitting recursion 30  --bsr30
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/0.5.7/underscore-min.js --location | md5sum
bddb511fbd4a54c1b77d7a26b8460c89  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.7.min.zopfli.js.gz | md5sum
bddb511fbd4a54c1b77d7a26b8460c89  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.5.7/underscore-min.js --location | sha1sum
34deaf3d4c79373b1921429f08843fb6a9f032e5  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.7.min.zopfli.js.gz | sha1sum
34deaf3d4c79373b1921429f08843fb6a9f032e5  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 3051 bytes bddb511fbd4a54c1b77d7a26b8460c89 (invalid)
cdnjs 2738 bytes bddb511fbd4a54c1b77d7a26b8460c89 (invalid)
cdnhttps 2730 bytes bddb511fbd4a54c1b77d7a26b8460c89 (invalid)

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 2751 bytes f4660dfb14a2433de87a681e88400ce4 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
2667 bytes -1 byte zopfli --i100000 --mls4096 --bsr30 --lazy --ohh February 3, 2016 @ 08:32
2668 bytes -1 byte zopfli --i10000 --mls4096 --bsr30 --lazy --ohh February 2, 2016 @ 23:13
2669 bytes -1 byte zopfli --i10000 --mls2 --bsr20 --lazy --ohh February 2, 2016 @ 23:10
2670 bytes -1 byte zopfli --i1000 --mls4096 --bsr18 --lazy --ohh February 2, 2016 @ 21:02
2671 bytes zopfli --i100 --mls8192 --bsr8 --lazy --ohh February 2, 2016 @ 20:57

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 @ 14:37.

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
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2667 2670 2670 2670
2669 2669 2669 2670 2669 2670 2670 2670 2670 2670 2670 2668 2670 2670 2670

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 2671 bytes 100%
1,000 2670 bytes -1 byte 100%
10,000 2668 bytes -2 bytes 100%
100,000 2667 bytes -1 byte 0.29%
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
2671 bytes +4 bytes (+0.15%)
2671 bytes +4 bytes (+0.15%)
2695 bytes +28 bytes (+1.05%) +24 bytes
2728 bytes +61 bytes (+2.29%) +57 bytes
2740 bytes +73 bytes (+2.74%) +69 bytes
2766 bytes +99 bytes (+3.71%) +95 bytes
2797 bytes +130 bytes (+4.87%) +126 bytes
2834 bytes +167 bytes (+6.26%) +163 bytes
2869 bytes +202 bytes (+7.57%) +198 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 2397 bytes -270 bytes (-10.12%)
Brotli (Wikipedia) brotli brotli -q 11 2462 bytes -205 bytes (-7.69%)
ZSTD (Wikipedia) zstd zstd -19 2684 bytes +17 bytes (+0.64%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 2722 bytes +55 bytes (+2.06%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 2745 bytes +78 bytes (+2.92%)
LZMA2 (Wikipedia) xz xz -9 2768 bytes +101 bytes (+3.79%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 3290 bytes +623 bytes (+23.36%)

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