( async() => {
  const items = 'abcdefghijklmnopqrstuvwxyz'.split('');
  const concurrency = 10;
  
  async function process() {
    return new Promise( r => setTimeout( r, Math.random() * 1000 ) );
  }
  
  async function report({ item, processed, total, done }){ 
    if ( done ) {
      return console.log( 'Processing complete (%d items)', total );
    }
    console.log( 'Processed %s (%d of %d)', item, processed, total );
  }
  
  await queue({ items, concurrency, process, report });
  
  console.log('Done');
})();