How would you overload the [] operator in javascript

You can do this with ES6 Proxy (available in all modern browsers)

var handler = {
    get: function(target, name) {
        return "Hello, " + name;
    }
};
var proxy = new Proxy({}, handler);

console.log(proxy.world); // output: Hello, world
console.log(proxy[123]); // output: Hello, 123

Check details on MDN.

You can't overload operators in JavaScript.

It was proposed for ECMAScript 4 but rejected.

I don't think you'll see it anytime soon.