Choose a version:
37% The original file has 22262 bytes (21.7k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 8206 bytes (8.0k, 37%).

After GZIP compression these minified files vary in size:
Boot
  2967 bytes (2.9k)
 CDN
gzip -6 (default)
  2690 bytes (2.6k)
 local copy
gzip -9
  2690 bytes (2.6k)
 local copy
Baidu
  2684 bytes (2.6k)
 CDN
cdnjs
  2672 bytes (2.6k)
 CDN
libdeflate -12
  2614 bytes (2.6k)
 local copy
7zip -mx=9 -tgzip
  2612 bytes (2.6k)
 local copy
zultra
  2612 bytes (2.6k)
 local copy
pigz -11 -n
  2611 bytes (2.5k)
 local copy
kzip -s0 -rn -b0
  2610 bytes (2.5k)
 local copy
Zopfli
  2607 bytes (2.5k)
 local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.2.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 0.5.2 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 65 bytes by using my UnderscoreJS 0.5.2 Zopfli version instead of the best available CDN (2.49% smaller than cdnjs, 2607 vs. 2672 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 --i100000 --mb8 --mls128 --bsr25 --lazy --ohh

(found February 10, 2019)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 128  --mls128
block splitting recursion 25  --bsr25
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/0.5.2/underscore-min.js --location | md5sum
10f714b13e1699993739cdb4006df282  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.2.min.zopfli.js.gz | md5sum
10f714b13e1699993739cdb4006df282  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/0.5.2/underscore-min.js --location | sha1sum
97ccdbc59522095a58a3bf33196a08c913437e4c  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-0.5.2.min.zopfli.js.gz | sha1sum
97ccdbc59522095a58a3bf33196a08c913437e4c  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 2967 bytes 10f714b13e1699993739cdb4006df282 (invalid)
cdnjs 2672 bytes 10f714b13e1699993739cdb4006df282 (invalid)

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 2684 bytes 89b0cf33d7210176b4c42e55faeb652f 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:

1.13.6, 1.13.5, 1.13.4, 1.13.3, 1.13.2, 1.13.1, 1.13.0,
1.12.1, 1.12.0,
1.11.0,
1.10.2, 1.10.1, 1.10.0,
1.9.2, 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
2607 bytes -1 byte zopfli --i100000 --mls128 --bsr25 --lazy --ohh February 10, 2019 @ 18:09
2608 bytes -1 byte zopfli --i10000 --mls512 --bsr14 --lazy --ohh February 2, 2016 @ 23:08
2609 bytes -1 byte zopfli --i1000 --mls2 --bsr20 --lazy --ohh February 2, 2016 @ 21:00
2610 bytes – zopfli --i100 --mls16384 --bsr10 --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 July 20, 2020 @ 12:48.

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 or 1,000,000.
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
2 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
4 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
6 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
7 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
8 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
9 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
10 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
11 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
12 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
13 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
14 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
15 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
16 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
17 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
18 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
19 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
20 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
21 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
22 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
23 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
25 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
30 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609
40 2609 2609 2609 2609 2608 2609 2607 2609 2608 2608 2608 2608 2609 2609 2609

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 2610 bytes – 100%
1,000 2609 bytes -1 byte 100%
10,000 2608 bytes -1 byte 100%
100,000 2607 bytes -1 byte 40.00%
1,000,000 2607 bytes – 0.29%
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
(any) 2610 bytes +3 bytes (+0.12%) –
1 2610 bytes +3 bytes (+0.12%) –
2 2631 bytes +24 bytes (+0.92%) +21 bytes
3 2669 bytes +62 bytes (+2.38%) +59 bytes
4 2681 bytes +74 bytes (+2.84%) +71 bytes
5 2708 bytes +101 bytes (+3.87%) +98 bytes
6 2737 bytes +130 bytes (+4.99%) +127 bytes
7 2770 bytes +163 bytes (+6.25%) +160 bytes
8 2807 bytes +200 bytes (+7.67%) +197 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 2333 bytes -274 bytes (-10.51%)
Brotli (Wikipedia) brotli brotli -q 11 2413 bytes -194 bytes (-7.44%)
Zstandard (Wikipedia) zstd zstd -19 2616 bytes +9 bytes (+0.35%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 2641 bytes +34 bytes (+1.30%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 2675 bytes +68 bytes (+2.61%)
LZMA2 (Wikipedia) xz xz -9 2700 bytes +93 bytes (+3.57%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 3228 bytes +621 bytes (+23.82%)

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 2024.
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

All trademarks are property of their respective owners. You know, the boring legal stuff.