Test your JavaScript on multiple engines with eshost-cli and jsvu

JavaScript engines are complex software components that optimize and run your JavaScript. Testing your code on multiple engines can be an effective way to understand how to write more performant libraries and applications. This is true no matter whether you are developing for the browser, Node.js, Deno, or for any other JavaScript runtime.

# Install JS engines with jsvu

Since most engines are written in languages like C, C++ and Rust, you’ll have to download the source files and compile them. However, you don’t have to do it yourself. A tool like jsvu can download and compile the most popular JS engines for you.

You can install the jsvu CLI with npm (or yarn):

npm install jsvu -g

and run it with:

jsvu

On the first run, jsvu asks you for the list of JavaScript engines you wish to manage through jsvu, then it creates:

1. a .jsvu directory in your home directory;
2. a status.json file inside of the .jsvu directory

Every time jsvu runs, including the first time, it installs the latest version of the engines specified in ~/.jsvu/status.json.

jsvu then compiles the engines’ source files and stores their binaries in ~/.jsvu/engines. It also creates either a symlink or a small shell script to execute each engine’s shell.

Having all JS engines in ~/.jsvu (through a symlink or a shell script) is quite convenient for setting up eshost-cli, the tool that runs your code on all of these engines.

# Run JS code on multiple engines with eshost-cli

You can install eshost-cli with npm (or yarn):

npm install -g eshost-cli

You add an ECMAScript host with the following syntax:

eshost --add <host name> <host type> <host path>

Let’s say that you have previously installed (with jsvu) GraalJS, SpiderMonkey and V8. Here is how you can add them as hosts:

eshost --add 'GraalJS' graaljs ~/.jsvu/graaljs
eshost --add 'SpiderMonkey' jsshell ~/.jsvu/spidermonkey
eshost --add 'V8' d8 ~/.jsvu/v8
eshost --add 'V8-debug' d8 ~/.jsvu/v8-debug

Now that you have configured some hosts, you can evaluate your code on multiple engines:

eshost --eval '1 + 2' # or just -e

#### GraalJS
3

#### SpiderMonkey
3

#### V8
3

#### V8-debug
3

If you prefer a tabular output, just pass the -t / --table flag:

eshost -e '1 + 2' --table # or just -t

┌────────────────┬───┐
│ GraalJS        │ 3 │
├────────────────┼───┤
│ SpiderMonkey   │ 3 │
├────────────────┼───┤
│ V8             │ 3 │
├────────────────┼───┤
│ V8-debug       │ 3 │
└────────────────┴───┘

You can evaluate your code on a single host with -h / --host:

eshost -e '1 + 2' -t -h GraalJS

┌────────────────┬───┐
│ GraalJS        │ 3 │
└────────────────┴───┘

You can also evaluate your code on multiple hosts, but first you need to group some hosts together, using a common tag.

Here is how to edit existing hosts:

eshost --edit 'V8' d8 ~/.jsvu/v8 --tags v8,latest
eshost --edit 'V8-debug' d8 ~/.jsvu/v8-debug --tags v8,debug

and how to select the hosts by tag:

eshost -e '1 + 2' -t --tags v8

┌────────────────┬───┐
│ V8             │ 3 │
├────────────────┼───┤
│ V8-debug       │ 3 │
└────────────────┴───┘

You can also pass arguments specific to the underlying JavaScript engine. This is useful if you want to understand how a particular engine works. You can output profiling data, enable tracing, generate debugging information from the engine garbage collector/s, optimizing compiler/s, etc.

For example, the flag --trace-gc-verbose can be useful if you want to understand how garbage collection works in V8.

eshost --edit 'V8-debug' --args '--trace-gc-verbose'

Keep in mind that every JS engine has its own flags, and there might be a lot of them. I think V8 has more than 500 flags!

Until now we have evaluated inline JavaScript, but eshost-cli allows you to pass a JavaScript file too.

Write the JavaScript you want to test…

// test.js
const x = 42;
console.log(`The answer is ${x}`);

…then run eshost-cli to evaluate it:

eshost test.js

Finally, when you no longer need a host, here is how to delete it:

eshost --delete 'JavaScriptCore'

• CLI
• JavaScript
• tests