Choose a version:
35% The original file has 24953 bytes (24.4k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 8690 bytes (8.5k, 35%).

After GZIP compression these minified files vary in size:
Boot
  3275 bytes (3.2k)
CDN
gzip -6 (default)
  2973 bytes (2.9k)
local copy
Baidu
  2969 bytes (2.9k)
CDN
gzip -9
  2967 bytes (2.9k)
local copy
cdnjs
  2956 bytes (2.9k)
CDN
unpkg
  2952 bytes (2.9k)
CDN
libdeflate -12
  2875 bytes (2.8k)
local copy
zultra
  2875 bytes (2.8k)
local copy
7zip -mx=9 -tgzip
  2872 bytes (2.8k)
local copy
kzip -s0 -rn -b0
  2872 bytes (2.8k)
local copy
pigz -11 -n
  2870 bytes (2.8k)
local copy
Zopfli
  2868 bytes (2.8k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.0.4.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 1.0.4 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 84 bytes by using my UnderscoreJS 1.0.4 Zopfli version instead of the best available CDN (2.93% smaller than unpkg, 2868 vs. 2952 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 --mls256 --bsr25 --lazy --ohh

(found February 11, 2019)
Description Value Parameter
iterations 100000  --i100000
maximum blocks 8  --mb8
maximum length score 256  --mls256
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/1.0.4/underscore-min.js --location | md5sum
cd539dbf84740fbaf28bb3e6750f646f  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.0.4.min.zopfli.js.gz | md5sum
cd539dbf84740fbaf28bb3e6750f646f  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.0.4/underscore-min.js --location | sha1sum
ee68308411c690cd51cc8a43fe6ae60d3d1678af  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.0.4.min.zopfli.js.gz | sha1sum
ee68308411c690cd51cc8a43fe6ae60d3d1678af  -

These CDNs send you the original file:
CDN Size (compressed) MD5 (uncompressed) Timestamp
Boot 3275 bytes cd539dbf84740fbaf28bb3e6750f646f (invalid)
cdnjs 2956 bytes cd539dbf84740fbaf28bb3e6750f646f (invalid)
unpkg 2952 bytes cd539dbf84740fbaf28bb3e6750f646f July 11, 2016 @ 16:49

And some CDNs send you a different file:
CDN Size (compressed) MD5 (uncompressed) Comment / Diff Timestamp
Baidu 2969 bytes e36a84958e3e3d8a03ddcbcf551189c3 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
2868 bytes -1 byte zopfli --i100000 --mls256 --bsr25 --lazy --ohh February 11, 2019 @ 15:07
2869 bytes zopfli --i100 --mls8 --bsr10 --lazy --ohh February 2, 2016 @ 19:07

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 @ 12:54.

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:
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
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869
2868 2868 2869 2869 2869 2869 2868 2868 2868 2869 2868 2869 2869 2869 2869

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 2869 bytes 100%
1,000 2869 bytes 100%
10,000 2869 bytes 100%
100,000 2868 bytes -1 byte 100%
1,000,000 2868 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
2872 bytes +4 bytes (+0.14%)
2872 bytes +4 bytes (+0.14%)
2897 bytes +29 bytes (+1.01%) +25 bytes
2929 bytes +61 bytes (+2.13%) +57 bytes
2937 bytes +69 bytes (+2.41%) +65 bytes
2969 bytes +101 bytes (+3.52%) +97 bytes
3002 bytes +134 bytes (+4.67%) +130 bytes
3038 bytes +170 bytes (+5.93%) +166 bytes
3074 bytes +206 bytes (+7.18%) +202 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 2577 bytes -291 bytes (-10.15%)
Brotli (Wikipedia) brotli brotli -q 11 2666 bytes -202 bytes (-7.04%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 2883 bytes +15 bytes (+0.52%)
Zstandard (Wikipedia) zstd zstd -19 2883 bytes +15 bytes (+0.52%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 2912 bytes +44 bytes (+1.53%)
LZMA2 (Wikipedia) xz xz -9 2960 bytes +92 bytes (+3.21%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 3456 bytes +588 bytes (+20.50%)

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 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.