concurrent-update-example.js

// Helper function to generate random delay
function randomWait() {
  return new Promise(resolve => {
    setTimeout(resolve, Math.ceil(Math.random() * 1000));
  });
}

// The "database"
const dataStore = {
  // Records Collection
  records: [
    { id: 1, foo: 'expedita odio fugiat', bar: 'eos natus pariatur' },
    { id: 2, foo: 'aspernatur reiciendis dolores', bar: 'illo delectus ut' },
    { id: 3, foo: 'rerum tempore minima', bar: 'qui hic dolor' }
  ],
  // Simulate an async get request
  get: async function get(collection, id) {
    const obj = this[collection].find(doc => doc.id === id);
    
    // Shallow clone doc to break reference to collection object
    return {...obj};
  },
  // Simulate an async save request
  save: async function save(collection, id, obj) {
    const index = this[collection].findIndex(doc => doc.id === obj.id);
    
    // "Store" record
    return this[collection][index] = { ...this[collection][index], ...obj };
  }
}

// Simulated two concurrent fetch and save requests
Promise.all([
  dataStore.get('records', 2).then(async record => {
    // Simulate a delay
    const wait = await randomWait();
    record.foo = 'Update';
    return dataStore.save('records', 2, record);
  }),
  dataStore.get('records', 2).then(async record => {
    // Simulate a delay
    const wait = await randomWait();
    record.bar = 'Update';
    return dataStore.save('records', 2, record);
  }),
]).then(
  results => Promise.all(results.concat(dataStore.get('records', 2)))
).then(
  ([firstUpdate, secondUpdate, curRecord]) => console.log({firstUpdate, secondUpdate, curRecord})
);