resolveClasses.ts

export const resolveClasses = (rules: {
  [className: string]: () => boolean
}, ...persistedClasses: string[]): string => {
  let classes: string[] = [];
  Object.keys(rules).forEach((className) => {
    const isResolved = rules[className]();
    if (isResolved) {
      classes.push(className);
    } else {
      // should remove it
      classes = classes.filter((needle) => (
        needle == className
      ));
    }
  });
  if (persistedClasses) {
    classes = classes.concat(persistedClasses);
  }
  return classes.join(' ');
}

// Usage

resolveClasses({
  'MyClass': () => true
  'IgnoreClass': () => false
}, 'AlwaysThere') // => 'MyClass AlwaysThere'