DataLoader

index.js

const nextTick = () => Promise.resolve();

class DataLoader {
  
  constructor( batch ) {
    this.batch = batch;
    this.queue = [];
    this.cache = new Map();
  }
  
  async load( key ) {
    if ( this.cache.has( key ) ) {
      return this.cache.get( key );
    }
    
    const promise = new Promise( ( resolve, reject ) => {
      this.queue.push({ key, resolve, reject });
      
      if ( this.queue.length === 1 ) {
        nextTick().then( () => dispatch( this ) );
      }
    });
    
    this.cache.set( key, promise );
    
    return promise;
  }
  
}

async function dispatch( loader ) {
  const queue = loader.queue;
  const cache = loader.cache;
  const keys  = loader.queue.map( ({ key }) => key );
  
  loader.queue = [];
  loader.cache = new Map;
  
  try {
    const values = await loader.batch( keys );
    
    queue.forEach( ( { key, resolve }, index ) => {
      resolve( values[ index ] );
    });
  } catch ( err ) {
    queue.forEach( ({ reject }) => reject( err ) );
  }
}

const data = [
  { id: 1, name: 'John' },
  { id: 2, name: 'Bob' },
  { id: 3, name: 'Tom' }
];

const loader = new DataLoader(
  async keys => {
    console.log('batch');
    return data.filter( d => keys.includes( d.id ) );
  }
);

Promise.all([
  loader.load( 1 ),
  loader.load( 2 ),
  loader.load( 3 ),
  loader.load( 3 )
]).then( console.log.bind( console ) );