ES6 Computed Property Names

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Object_initializer#Computed_property_names

// Computed property names (ES2015)
let i = 0
let a = {
  ['foo' + ++i]: i,
  ['foo' + ++i]: i,
  ['foo' + ++i]: i
}

console.log(a.foo1) // 1
console.log(a.foo2) // 2
console.log(a.foo3) // 3