const BUTTON_TYPE_CLASSES = {
  base : 'base',
  inverted : 'inverted'
}

const getButton = (buttonType) =>
({
  [BUTTON_TYPE_CLASSES.base] : BaseComponent,
  [BUTTON_TYPE_CLASSES.inverted] : InvertedComponent,
}[buttonType]);

I understand what this code blocks mean. But I don't know what works brackets in object of function getButton.

Why don't use just BUTTON_TYPE_CLASSES.base : BaseComponent... ? Why use [BUTTON_TYPE_CLASSES.base] : BaseComponent... ?