Some benefits over Mozilla Spidermonkey:

 * Small. We're using 6 or so C files vs 700+ SM91 C++ files.

 * Built with Apache CouchDB as opposed having to maintain a separate SM
   package, like for RHEL9, for instance, where they dropped support for SM
   already [1].

 * Embedding friendly. Designed from ground-up for embedding. SM has been
   updating the C++ API such that we have to keep copy-pasting new versions of
   our C++ code every year or so [2].

 * Easy to modify to accept Spidermonkey 1.8.5 top level functions for
   map/reduce code so we don't have have to parse the JS, AST transform it, and
   then re-compile it.

 * Better sandboxing. Creating a whole JSRuntime takes only 300 microseconds, so
   we can afford to do that on reset. JSRuntimes cannot share JS data or object
   between them.

 * Seems to be faster in preliminary benchmarking with small
   concurrent VDU and view builds:
     https://gist.github.com/nickva/ed239651114794ebb138b1f16c5f6758
   Results seem promising:
     - 4x faster than SM 1.8.5
     - 5x faster than SM 91
     - 6x reduced memory usage per couchjs process (5MB vs 30MB)

 * Allows compiling JS bytecode ahead of time a C array of bytes.

QuickJS can be built alongside Spidermonkey and toggled on/off at runtime:

```
./configure --dev --js-engine=quickjs
```

This makes it the default engine. But Spidermonkey can still be set in the
config option.

```
[couchdb]
js_engine = spidermonkey | quickjs
```

To test individual views, without switching the default use the
`javascript_quickjs` language in the design docs. To keep using Spidermonkey
engine after switching the default, can use `javascript_spidermonkey` language
in design docs. However, language selection will reset the view and the view
will have to be rebuilt.

It's also possible to build without Spidermonkey support completely by using:
```
./configure --disable-spidermonkey
```

Issue: https://github.com/apache/couchdb/issues/4448

[1] https://github.com/apache/couchdb/issues/4154
[2] https://github.com/apache/couchdb/pull/4305