What does it mean by live bindings?

Live bindings is a concept introduced in ES modules. It means that when the exporting module changes a value, the change will be visible from the importer side. This is not the case for CommonJS modules. Module exports are copied in CommonJS. Hence importing modules cannot see changes happened on the exporter side.

ESM

counter.mjs

export let count = 1;
export function increment() {
    ++count;
}

index.mjs

import { count, increment } from './counter.mjs';
console.log(count);
increment();
console.log(count);

Output

$ node --experimental-modules index.mjs
1
2

CJS

counter.js

let count = 1;
function increment() {
    ++count;
}

exports.count = count;
exports.increment = increment;

index.js

const { count, increment } = require('./counter.js');
console.log(count);
increment();
console.log(count);

Output

$ node index.js
1
1

More resources on the topic:

• What do ES6 modules export? by Axel Rauschmayer
• ES modules: A cartoon deep-dive by Lin Clark
• Chapter on Modules in Exploring JS by Axel Rauschmayer

in CJS, require("./foo") gets you a reference to the exports object of foo.js.

1. let's set exports.x to 0 in foo.js

exports.x = 0

2. In bar.js we get a reference to foo's exports and mutate it:

const fooExports = require("./foo")
fooExports.x += 1

Now we put everything in action in main.js

const fooExports = require("./foo") // getting a reference to exports
console.log(fooExports.x) // x is indeed 0
require("./bar") // we execute bar.js that mutate exports
console.log(fooExports.x) // x is now 1, because fooExports is a reference to exports

TLDR: What we receive from require is not a copy. It's a reference to the exports object.