ES6 all the things

async-array.js

import { EventEmitter } from 'events';

export class AsyncArray extends EventEmitter {
    constructor(fetch, maxSize = 1000, pageSize = 100) {
        super();

        this.cache = new Map();
        this.requestsInProgress = new Set();
        this.fetch = fetch;
        this.maxSize = maxSize;
        this.pageSize = pageSize;
    }

    get(index) {
        // try to get from this.cache
        if (this.cache.has(index)) {
            return this.cache.get(index);
        } else {
            // fetch page containing requested index
            let pageStart = Math.floor(index / this.pageSize) * this.pageSize;
            let requestKey = `${pageStart}-${this.pageSize}`;
            if (!this.requestsInProgress.has(requestKey)) {
                this.requestsInProgress.add(requestKey);
                this.fetch(pageStart, this.pageSize)
                    // cache items returned
                    .then(data => data.forEach((item, itemIndex) => this._cacheItem(itemIndex + pageStart, item)))
                    .then(() => this.emit('change'))
                    .always(() => this.requestsInProgress.delete(requestKey));
            }

            // return empty placeholder
            return null;
        }
    }

    _cacheItem(index, item) {
        // add item to cache
        this.cache.set(index, item);

        // drop oldest data from cache as needed
        if (this.cache.length > this.maxSize) {
            let dropCount = this.cache.length - this.maxSize;
            let itemCount = 0;
            for (let cachedItem of this.cache.keys()) {
                if (itemCount < dropCount) {
                    this.cache.delete(cachedItem);
                    itemCount++;
                } else {
                    break;
                }
            }
        }
    }
}