Choose a version:
31% The original file has 52919 bytes (51.7k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 16449 bytes (16.1k, 31%).

After GZIP compression these minified files vary in size:
Boot
  6574 bytes (6.4k)
CDN, direct link
jsdelivr
  6574 bytes (6.4k)
CDN, direct link
gzip -6 (default)
  5869 bytes (5.7k)
local copy
cdnjs
  5862 bytes (5.7k)
CDN, direct link
gzip -9
  5856 bytes (5.7k)
local copy
cdnhttps
  5845 bytes (5.7k)
CDN, direct link
fastcdn
  5843 bytes (5.7k)
CDN, direct link
unpkg
  5843 bytes (5.7k)
CDN, direct link
Yandex
  5835 bytes (5.7k)
CDN, direct link
libdeflate -12
  5684 bytes (5.6k)
local copy
7zip -mx=9 -tgzip
  5679 bytes (5.5k)
local copy
pigz -11 -n
  5673 bytes (5.5k)
local copy
kzip -s0 -rn -b1
  5672 bytes (5.5k)
local copy
Zopfli
  5664 bytes (5.5k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.8.3.min.js

You will automatically get the smallest UnderscoreJS 1.8.3 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 171 bytes by using my UnderscoreJS 1.8.3 Zopfli version instead of the best available CDN (3.02% smaller than Yandex, 5664 vs. 5835 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 --i10000000 --mb8 --mls256 --bsr17 --lazy --ohh

(found November 11, 2015)
Description Value Parameter
iterations 10000000  --i10000000
maximum blocks 8  --mb8
maximum length score 256  --mls256
block splitting recursion 17  --bsr17
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/1.8.3/underscore-min.js --location | md5sum
543feb1ecaf06ea516f8cec5f9f3f279  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.8.3.min.zopfli.js.gz | md5sum
543feb1ecaf06ea516f8cec5f9f3f279  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.8.3/underscore-min.js --location | sha1sum
2a515632e0fd8ffdeb8d94cb25e44cf287feb32f  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.8.3.min.zopfli.js.gz | sha1sum
2a515632e0fd8ffdeb8d94cb25e44cf287feb32f  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 6574 bytes 543feb1ecaf06ea516f8cec5f9f3f279 (invalid)
jsdelivr 6574 bytes 543feb1ecaf06ea516f8cec5f9f3f279 (invalid)
cdnjs 5862 bytes 543feb1ecaf06ea516f8cec5f9f3f279 (invalid)
cdnhttps 5845 bytes 543feb1ecaf06ea516f8cec5f9f3f279 December 24, 2015 @ 07:36
fastcdn 5843 bytes 543feb1ecaf06ea516f8cec5f9f3f279 November 5, 2015 @ 05:55
unpkg 5843 bytes 543feb1ecaf06ea516f8cec5f9f3f279 July 11, 2016 @ 16:48
Yandex 5835 bytes 543feb1ecaf06ea516f8cec5f9f3f279 December 22, 2015 @ 17:09

Note: only the MD5 hashes are shown to keep things simple.

Other Versions

Available UnderscoreJS versions at minime.stephan-brumme.com:

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 and ThreeJS.

Changelog

Best Zopfli parameters so far:
Size Improvement Parameters Found
5664 bytes -1 byte zopfli --i10000000 --mls256 --bsr17 --lazy --ohh November 11, 2015 @ 21:13
5665 bytes -2 bytes zopfli --i100000 --mls256 --bsr17 --lazy --ohh September 22, 2015 @ 20:02
5667 bytes -1 byte zopfli --i10000 --mls4096 --bsr6 --lazy --ohh September 8, 2015 @ 11:05
5668 bytes -1 byte zopfli --i1000 --mls4096 --bsr6 --lazy --ohh September 8, 2015 @ 10:37
5669 bytes zopfli --i100 --mls8192 --bsr10 --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 @ 16:48.

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, 100,000, 1,000,000 or 10,000,000.
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
5671 5670 5680 5670 5670 5670 5670 5671 5670 5670 5670 5670 5669 5669 5669
5670 5670 5675 5670 5670 5675 5668 5670 5669 5670 5670 5668 5669 5668 5668
5680 5676 5671 5670 5677 5677 5671 5671 5669 5670 5670 5667 5667 5667 5667
5676 5674 5675 5670 5677 5674 5671 5672 5669 5670 5670 5667 5674 5672 5672
5673 5670 5675 5670 5677 5675 5671 5672 5669 5670 5670 5670 5669 5667 5667
5680 5675 5679 5670 5677 5670 5670 5672 5669 5670 5670 5670 5667 5667 5667
5680 5675 5675 5670 5677 5676 5678 5677 5669 5670 5670 5670 5667 5667 5667
5670 5670 5675 5670 5670 5670 5670 5672 5668 5670 5670 5670 5672 5671 5671
5673 5674 5677 5670 5677 5674 5671 5671 5669 5670 5670 5667 5669 5670 5670
5680 5674 5675 5670 5676 5670 5671 5670 5670 5670 5670 5670 5667 5667 5667
5672 5672 5674 5671 5670 5670 5671 5671 5669 5670 5670 5667 5668 5668 5668
5673 5677 5674 5670 5676 5675 5671 5679 5669 5670 5670 5667 5674 5667 5667
5680 5674 5675 5670 5676 5674 5670 5668 5669 5670 5670 5670 5667 5667 5667
5675 5674 5675 5670 5676 5675 5671 5664 5669 5670 5670 5670 5670 5667 5667
5674 5674 5675 5670 5677 5675 5670 5669 5669 5670 5670 5670 5667 5667 5667
5680 5680 5679 5670 5677 5675 5671 5677 5669 5670 5670 5670 5670 5667 5667
5674 5674 5679 5670 5677 5675 5671 5673 5669 5670 5670 5667 5669 5667 5667
5675 5676 5675 5670 5677 5675 5670 5677 5668 5670 5670 5670 5670 5667 5667
5672 5672 5679 5670 5678 5675 5671 5674 5669 5670 5670 5670 5667 5667 5667
5680 5674 5675 5670 5677 5675 5670 5669 5669 5670 5670 5670 5670 5667 5667
5674 5674 5675 5670 5677 5675 5671 5674 5669 5670 5670 5669 5667 5668 5668
5679 5675 5675 5670 5676 5675 5672 5672 5670 5670 5670 5667 5668 5668 5668
5679 5674 5675 5670 5676 5677 5672 5672 5670 5670 5670 5667 5668 5667 5667

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 5669 bytes 100%
1,000 5667 bytes -2 bytes 100%
10,000 5667 bytes 100%
100,000 5665 bytes -2 bytes 69.28%
1,000,000 5665 bytes 2.03%
10,000,000 5664 bytes -1 byte 0.29%

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
5672 bytes +8 bytes (+0.14%)
5672 bytes +8 bytes (+0.14%)
5696 bytes +32 bytes (+0.56%) +24 bytes
5727 bytes +63 bytes (+1.11%) +55 bytes
5746 bytes +82 bytes (+1.45%) +74 bytes
5755 bytes +91 bytes (+1.61%) +83 bytes
5789 bytes +125 bytes (+2.21%) +117 bytes
5829 bytes +165 bytes (+2.91%) +157 bytes
5856 bytes +192 bytes (+3.39%) +184 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 4996 bytes -668 bytes (-11.79%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 5207 bytes -457 bytes (-8.07%)
Brotli (Wikipedia) brotli brotli -q 11 5302 bytes -362 bytes (-6.39%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 5576 bytes -88 bytes (-1.55%)
ZSTD (Wikipedia) zstd zstd -19 5676 bytes +12 bytes (+0.21%)
LZMA2 (Wikipedia) xz xz -9 5724 bytes +60 bytes (+1.06%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 5782 bytes +118 bytes (+2.08%)

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 2017.
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