function falsy(value) {
  return !value;
}
Returns `true` if the value is falsy - if I recall correctly, '', 0, false,
undefined, NaN or null. I don't know if we should stick to the default JS
behavior or attempt to replicate what Python believes should be falsy (i.e.,
empty arrays, empty dicts, not 0...).
@param { any } value
@returns { boolean }