It is actually ES5: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Refe...

In a nutshell: it is used for computed properties, which compute their value on 'get' operations instead of storing them.

No, just not commonly used; getters and setters aren't terribly useful in a language that allows such easy direct modification of objects (as opposed to say Java)

But transparent getters and setters can be used to emulate private or immutable properties:

    function Foo() {
      var _bar = 42;
      return {
        get bar() {
          return _bar;
        }
      } 
    }

    var foo = Foo();
    foo.bar
    /*
    42
    */

However the fact that you can transparently replace a simple property by a computed one (e.g. for backwards compatibility or to trigger side effects in observables) does make explicit, Java-style, getters and setters less useful.