Choose a version:
32% The original file has 41344 bytes (40.4k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 13432 bytes (13.1k, 32%).

After GZIP compression these minified files vary in size:
Boot
  5142 bytes (5.0k)
 CDN
jsdelivr
  5142 bytes (5.0k)
 CDN
gzip -6 (default)
  4627 bytes (4.5k)
 local copy
Baidu
  4616 bytes (4.5k)
 CDN
cdnjs
  4616 bytes (4.5k)
 CDN
gzip -9
  4613 bytes (4.5k)
 local copy
unpkg
  4607 bytes (4.5k)
 CDN
cdnhttps
  4603 bytes (4.5k)
 CDN
7zip -mx=9 -tgzip
  4490 bytes (4.4k)
 local copy
libdeflate -12
  4488 bytes (4.4k)
 local copy
pigz -11 -n
  4488 bytes (4.4k)
 local copy
Zopfli
  4485 bytes (4.4k)
 local copy
kzip -s0 -rn -b1
  4484 bytes (4.4k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.4.3.min.js

You will automatically get the smallest UnderscoreJS 1.4.3 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 118 bytes by using my UnderscoreJS 1.4.3 Zopfli version instead of the best available CDN (2.63% smaller than cdnhttps, 4485 vs. 4603 bytes):
Download
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 --mls512 --bsr2 --lazy --ohh

(found September 8, 2015)
Description Value Parameter
iterations 1000  --i1000
maximum blocks 8  --mb8
maximum length score 512  --mls512
block splitting recursion 2  --bsr2
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/1.4.3/underscore-min.js --location | md5sum
45635c8658599ecae698d0d45efc480d  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.4.3.min.zopfli.js.gz | md5sum
45635c8658599ecae698d0d45efc480d  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.4.3/underscore-min.js --location | sha1sum
8438e159195ef97e07bd1c7fe16c7ffdd308eb58  -
curl --silent --compressed http://minime.stephan-brumme.com/files/underscorejs/underscore-1.4.3.min.zopfli.js.gz | sha1sum
8438e159195ef97e07bd1c7fe16c7ffdd308eb58  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 5142 bytes 45635c8658599ecae698d0d45efc480d (invalid)
jsdelivr 5142 bytes 45635c8658599ecae698d0d45efc480d (invalid)
Baidu 4616 bytes 45635c8658599ecae698d0d45efc480d (invalid)
cdnjs 4616 bytes 45635c8658599ecae698d0d45efc480d (invalid)
unpkg 4607 bytes 45635c8658599ecae698d0d45efc480d July 11, 2016 @ 16:49
cdnhttps 4603 bytes 45635c8658599ecae698d0d45efc480d December 24, 2015 @ 07:36

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
4485 bytes -1 byte zopfli --i1000 --mls512 --bsr2 --lazy --ohh September 8, 2015 @ 10:48
4486 bytes – zopfli --i100 --mls1024 --bsr11 --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 @ 16:49.

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
2 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4491 4491
4 4487 4487 4486 4485 4485 4485 4485 4485 4485 4485 4485 4485 4486 4487 4487
6 4487 4487 4486 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485
7 4487 4487 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4486 4490 4490
8 4487 4487 4487 4485 4485 4485 4485 4485 4485 4485 4485 4485 4490 4490 4490
9 4487 4487 4486 4486 4485 4485 4485 4485 4485 4485 4485 4485 4486 4485 4485
10 4494 4494 4489 4486 4485 4485 4485 4485 4485 4485 4485 4485 4486 4488 4488
11 4487 4487 4486 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485
12 4495 4495 4489 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4486 4486
13 4494 4494 4489 4485 4485 4485 4485 4485 4485 4485 4485 4485 4486 4488 4488
14 4495 4495 4486 4485 4485 4485 4485 4485 4485 4485 4485 4485 4486 4486 4486
15 4487 4487 4485 4486 4485 4485 4485 4485 4485 4485 4485 4485 4486 4488 4488
16 4486 4486 4488 4486 4485 4485 4485 4485 4485 4485 4485 4485 4486 4485 4485
17 4487 4487 4486 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485
18 4486 4486 4489 4485 4485 4485 4485 4485 4485 4485 4485 4485 4486 4485 4485
19 4487 4487 4486 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485
20 4487 4487 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485
21 4494 4494 4494 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485
22 4487 4487 4486 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485
23 4486 4486 4486 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4486 4486
25 4487 4487 4485 4486 4485 4485 4485 4485 4485 4485 4485 4485 4487 4486 4486
30 4487 4487 4487 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4486 4486
40 4487 4487 4486 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485 4485

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 4486 bytes – 100%
1,000 4485 bytes -1 byte 100%
10,000 4485 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
0 4484 bytes -1 byte (-0.02%) –
1 4484 bytes -1 byte (-0.02%) –
2 4506 bytes +21 bytes (+0.47%) +22 bytes
3 4532 bytes +47 bytes (+1.05%) +48 bytes
4 4546 bytes +61 bytes (+1.36%) +62 bytes
5 4577 bytes +92 bytes (+2.05%) +93 bytes
6 4609 bytes +124 bytes (+2.76%) +125 bytes
7 4645 bytes +160 bytes (+3.57%) +161 bytes
8 4675 bytes +190 bytes (+4.24%) +191 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 3969 bytes -516 bytes (-11.51%)
Brotli (Wikipedia) brotli brotli -q 11 4180 bytes -305 bytes (-6.80%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 4384 bytes -101 bytes (-2.25%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 4499 bytes +14 bytes (+0.31%)
ZSTD (Wikipedia) zstd zstd -19 4499 bytes +14 bytes (+0.31%)
LZMA2 (Wikipedia) xz xz -9 4572 bytes +87 bytes (+1.94%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 4823 bytes +338 bytes (+7.54%)

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):
Load and Parse Data


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