Async Concurrent Queue

example.js

( 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');
})();

queue.js

async function queue({ items, concurrency = 5, process, report = () => {} }) {
  const pending = items.slice();
  const total   = pending.length;
  
  const state = {
    total: pending.length,
    inflight: 0,
    processed: 0,
    queued: pending.length,
    done: false
  };
  
  return new Promise( ( resolve, reject ) => {
    async function dequeue() {
      if ( pending.length === 0 && state.inflight === 0 ) {
        state.done = true;
        report({ item: null, ...state });
        return setImmediate( resolve );
      }
      
      if ( state.inflight >= concurrency || pending.length === 0 ) {
        return;
      }
      
      const item = pending.shift();
      
      state.inflight++;
      state.queued--;
      
      process( item ).then( () => {
        state.inflight--;
        state.processed++;
        report({ item, ...state });
        dequeue();
      }, reject );

      dequeue();
    }
    
    dequeue();
  });
}