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.

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