getClassNames.ts

type ClassNameObject = { [key: string]: boolean };

type ClassNameList = Array<(string | ClassNameObject)>;

type ClassNameArguments = (string | ClassNameObject | ClassNameList);

type ClassNameEntry = [keyof ClassNameObject, ClassNameObject[keyof ClassNameObject]];

const isTruthyEntryValue = ([ , value ]: ClassNameEntry) => Boolean(value);

const getEntryKey = ([ key ]: ClassNameEntry) => key;

const toClassNames = ( value ): string | Array<string> => {
  if ( typeof value === 'string' )
    return value;
  if ( Array.isArray( value ) && value.length )
    return [].concat( ...value.map( toClassNames ) );
  if ( value && typeof value === 'object' )
    return Object.entries( value ).filter( isTruthyEntryValue ).map( getEntryKey );
}

const getClassNames = ( ...values: Array<ClassNameArguments> ) => (
  [  ].concat(
    ...values.map( toClassNameList )
  ).join( ' ' );
)