Choose a version:
29% The original file has 60061 bytes (58.7k) and is available from the project website.
There you can find the official minified version, too, which brings down the size to 17532 bytes (17.1k, 29%).

After GZIP compression these minified files vary in size:
unpkg
  7475 bytes (7.3k)
CDN
gzip -6 (default)
  6730 bytes (6.6k)
local copy
gzip -9
  6728 bytes (6.6k)
local copy
7zip -mx=9 -tgzip
  6565 bytes (6.4k)
local copy
libdeflate -12
  6562 bytes (6.4k)
local copy
kzip -s0 -rn -b1
  6554 bytes (6.4k)
local copy
pigz -11 -n
  6554 bytes (6.4k)
local copy
zultra
  6553 bytes (6.4k)
local copy
Zopfli
  6532 bytes (6.4k)
local copy

perma-link to the smallest file on my server:
http://minime.stephan-brumme.com/files/underscorejs/underscore-1.10.1.min.js (or via HTTPS)

You will automatically get the smallest UnderscoreJS 1.10.1 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 943 bytes by using my UnderscoreJS 1.10.1 Zopfli version instead of the best available CDN (14.44% smaller than unpkg, 6532 vs. 7475 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 --i1000000 --mb8 --mls8192 --bsr23 --lazy --ohh

(found May 19, 2020)
Description Value Parameter
iterations 1000000  --i1000000
maximum blocks 8  --mb8
maximum length score 8192  --mls8192
block splitting recursion 23  --bsr23
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/1.10.1/underscore-min.js --location | md5sum
6f4bb6d1c719d3607de8cf6d3b4b851f  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.10.1.min.zopfli.js.gz | md5sum
6f4bb6d1c719d3607de8cf6d3b4b851f  -

SHA1:
curl --silent --compressed https://raw.githubusercontent.com/jashkenas/underscore/1.10.1/underscore-min.js --location | sha1sum
f59e6eb3de8cfb0cf0edd6dcec46fbcac2976bd1  -
curl --silent --compressed https://minime.stephan-brumme.com/files/underscorejs/underscore-1.10.1.min.zopfli.js.gz | sha1sum
f59e6eb3de8cfb0cf0edd6dcec46fbcac2976bd1  -

All listed CDNs deliver identical contents:
CDN Size (compressed) MD5 (uncompressed) Timestamp
unpkg 7475 bytes 6f4bb6d1c719d3607de8cf6d3b4b851f (invalid)

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

Other Versions

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

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
6532 bytes -1 byte zopfli --i1000000 --mls8192 --bsr23 --lazy --ohh May 19, 2020 @ 22:27
6533 bytes -1 byte zopfli --i100000 --mls32768 --bsr18 --lazy --ohh May 19, 2020 @ 15:29
6534 bytes -1 byte zopfli --i10000 --mls32768 --bsr18 --lazy --ohh May 19, 2020 @ 14:41
6535 bytes -2 bytes zopfli --i1000 --mls32768 --bsr18 --lazy --ohh May 19, 2020 @ 14:36
6537 bytes zopfli --i100 --mls32768 --bsr17 --lazy --ohh May 19, 2020 @ 14:30

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:
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
6553 6553 6554 6555 6553 6547 6547 6552 6549 6552 6553 6550 6553 6553 6553
6551 6552 6551 6552 6552 6543 6540 6543 6549 6549 6542 6554 6554 6535 6535
6544 6545 6550 6549 6549 6550 6548 6549 6549 6548 6543 6534 6554 6536 6536
6548 6537 6549 6546 6550 6537 6549 6550 6549 6549 6539 6551 6554 6533 6533
6552 6552 6549 6550 6553 6549 6554 6544 6544 6550 6545 6542 6553 6534 6534
6547 6554 6549 6546 6552 6542 6550 6550 6549 6549 6543 6537 6536 6534 6534
6548 6548 6552 6546 6550 6549 6551 6551 6542 6548 6543 6553 6553 6533 6533
6554 6553 6549 6549 6550 6545 6550 6550 6549 6548 6545 6553 6553 6542 6542
6555 6555 6555 6554 6537 6537 6540 6550 6549 6548 6549 6539 6553 6533 6533
6549 6549 6549 6549 6550 6550 6550 6548 6550 6549 6553 6534 6554 6553 6553
6553 6553 6543 6542 6542 6543 6542 6545 6549 6539 6548 6536 6536 6532 6532
6548 6548 6549 6549 6550 6539 6549 6550 6549 6549 6539 6551 6533 6553 6553
6548 6554 6549 6549 6534 6538 6537 6550 6549 6544 6539 6540 6541 6536 6536
6554 6553 6551 6549 6550 6536 6551 6552 6549 6549 6539 6542 6554 6533 6533
6548 6548 6548 6550 6552 6537 6537 6545 6549 6548 6545 6537 6553 6533 6533
6549 6553 6548 6549 6550 6537 6551 6548 6549 6548 6539 6553 6538 6534 6534
6547 6550 6549 6550 6552 6538 6539 6550 6548 6548 6539 6538 6553 6553 6553
6554 6554 6551 6549 6550 6550 6551 6550 6549 6543 6554 6541 6539 6539 6539
6549 6549 6549 6547 6553 6538 6537 6550 6549 6548 6543 6539 6553 6532 6532
6554 6554 6549 6549 6550 6550 6551 6550 6548 6548 6553 6540 6532 6532 6532
6554 6554 6549 6552 6550 6538 6549 6550 6549 6548 6546 6535 6553 6534 6534
6547 6552 6550 6550 6550 6549 6538 6551 6549 6549 6539 6540 6553 6536 6536
6548 6549 6549 6549 6550 6549 6551 6548 6550 6549 6539 6540 6532 6532 6532

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 6537 bytes 100%
1,000 6535 bytes -2 bytes 100%
10,000 6534 bytes -1 byte 100%
100,000 6533 bytes -1 byte 10.14%
1,000,000 6532 bytes -1 byte 6.09%
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
6554 bytes +22 bytes (+0.34%)
6554 bytes +22 bytes (+0.34%)
6568 bytes +36 bytes (+0.55%) +14 bytes
6593 bytes +61 bytes (+0.93%) +39 bytes
6611 bytes +79 bytes (+1.21%) +57 bytes
6616 bytes +84 bytes (+1.29%) +62 bytes
6644 bytes +112 bytes (+1.71%) +90 bytes
6682 bytes +150 bytes (+2.30%) +128 bytes
6708 bytes +176 bytes (+2.69%) +154 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 - for example, your browser actually supports it !
Algorithm Program Parameters Size Compared To Best Zopfli
RAR (proprietary) RAR rar a -m5 -md64m -mc63:128t -mt1 5858 bytes -674 bytes (-10.32%)
Brotli (Wikipedia) brotli brotli -q 11 6100 bytes -432 bytes (-6.61%)
PPMd (Wikipedia) 7zip 7za a -mx=9 -m0=ppmd 6112 bytes -420 bytes (-6.43%)
Zstandard (Wikipedia) zstd zstd -19 6524 bytes -8 bytes (-0.12%)
LZMA2 (Wikipedia) xz xz -9 6540 bytes +8 bytes (+0.12%)
Burrows-Wheeler transform (Wikipedia) bzip2 bzip2 -9 6557 bytes +25 bytes (+0.38%)
ZPAQ (Wikipedia) zpaq zpaq -method 69 6634 bytes +102 bytes (+1.56%)

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 2020.
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.