bemify.js

export function bemify(block, modifiers) {
  return block + ' ' + modifiersToArray(modifiers)
    .map(modifier => `${block}--${modifier}`)
    .join(' ');
}

function modifiersToArray(modifiers) {
  if (!modifiers) {
    return []
  }
  
  if (Array.isArray(modifiers)) {
    return modifiers
  }

  if (typeof modifiers == 'string') {
    return modifiers.split(' ')
  }

  return Object.keys(modifiers).filter(modifier => modifiers[modifier])
}

console.log(bemify('button', null))
// button

console.log(bemify('button', []))
// button

console.log(bemify('button', 'red close'))
// button button--red button--close

console.log(bemify('button', ['red', 'close']))
// button button--red button--close

console.log(bemify('button', {
  red: true,
  close: true,
  nope: false
}))
// button button--red button--close