Flow + functional programming = 💩

flow-fp.js

// Well, shit
const verticalModifiers = ['top', 'middle', 'bottom'];
const horizontalModifiers = ['start', 'center', 'end'];
const modifiers = verticalModifiers.concat(horizontalModifiers);
const getPassthroughClassNames = () => modifiers
  .map(name => [name, props[name]])
  .filter(([, value]) => value)
  .map(([name, value]) => `${name}-${value}`)
  .map(getClass);