How does the Node.js REPL display previews?
When typing in the node repl,
you get instant previews as you type,
like this:
Clearly, the REPL is executing your code every time you hit a key.
But how can this be safe?!
Imagine the chaos if you typed rm -rf /foo/bar/baz
and your shell tried to execute it at every keystroke!
The Node.js REPL implementation uses the inspector module,
which is an interface to V8.
Here’s a basic REPL for Node.js:
const readline = require('readline');
const inspector = require('inspector');
const session = new inspector.Session();
session.connect();
const rl = readline.createInterface({ input: process.stdin });
rl.on('line', function(line) {
session.post(
'Runtime.evaluate',
{ expression: line },
function cb(err, res) {
console.log(res.result.value);
}
);
});
The key is that, for previews,
the REPL passes throwOnSideEffect: true to V8.
I’m not sure exactly what V8 considers a side-effect,
but apparently it doesn’t consider Math.random() to have side-effects.
I’m like 99% sure it would consider “deleting all my files” to be a side-effect, though.
Tagged #programming, #javascript. All content copyright James Fisher 2020. This post is not associated with my employer.
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