Perhaps more often than not, property names in an object are based on something dynamic rather than a static value. In ES5, you may end up with code like the following to build an object with dynamic property names:

var dynamic1 = 'hello',
  dynamic2 = 'world';
var obj = {};
var propName1 = dynamic1 + dynamic2;
var propName2 = 5 * 11;
obj[propName1] = true;
obj[propName2] = false;

> obj
// { 55: false, 'helloworld': true }

With ES6, we get computed property names which allow us to do all of these inline when defining the object. We just have to wrap the code that will be evaluated to a property name in brackets:

const dynamic1 = 'hello',
  dynamic2 = 'world';
const obj = {
  [dynamic1 + dynamic2]: true,
  [5 * 11]: false
};

> obj
// { 55: false, 'helloworld': true }

Computed properties are already available in many modern browsers.