Example on how to achieve RxJS observable caching and storage in Angular 2+. Ideal for storing Http requests client side for offline usage.

example.component.ts

import { Component, OnInit, OnDestroy } from '@angular/core';
import {Http} from "@angular/http";
import { LocalCacheService } from "./local-cache.service";

@Component({
  selector: 'app-example',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.scss']
})
export class ExampleComponent implements OnInit, OnDestroy {

  constructor(public http:Http, public cache:LocalCacheService){
    
    //Cache an observable
    let requestObservable = this.http.get("http://example.com/path/to/api").map(res => res.json())

    this.cache.observable('my-cache-key', requestObservable, 300).subscribe(result => {
        //Use result
        console.log(result)
    });
  }
}

local-cache.service.ts

import {Injectable} from "@angular/core";
import {LocalStorageService} from "./local-storage.service";
import {Observable} from "rxjs/Observable";
import {isEmpty, isString, isNumber, isDate} from 'lodash';

@Injectable()
export class LocalCacheService {

  /**
   * Default expiry in seconds
   *
   * @type {number}
   */
  defaultExpires: number = 86400; //24Hrs

  constructor(private localstorage: LocalStorageService) {}

  /**
   * Cache or use result from observable
   *
   * If cache key does not exist or is expired, observable supplied in argument is returned and result cached
   *
   * @param key
   * @param observable
   * @param expires
   * @returns {Observable<T>}
   */
  public observable<T>(key: string, observable: Observable<T>, expires:number = this.defaultExpires): Observable<T> {
    //First fetch the item from localstorage (even though it may not exist)
    return this.localstorage.getItem(key)
      //If the cached value has expired, nullify it, otherwise pass it through
      .map((val: CacheStorageRecord) => val && (new Date(val.expires)).getTime() > Date.now() ? val : null)
      //At this point, if we encounter a null value, either it doesnt exist in the cache or it has expired.
      //If it doesnt exist, simply return the observable that has been passed in, caching its value as it passes through
      .flatMap((val: CacheStorageRecord | null) => {
        if (!isEmpty(val)) {
          return Observable.of(val.value);
        } else {
          return observable.flatMap((val:any) => this.value(key, val, expires)); //The result may have 'expires' explicitly set
        }
      })
  }

  /**
   * Cache supplied value until expiry
   *
   * @param key
   * @param value
   * @param expires
   * @returns {Observable<T>}
   */
  value<T>(key:string, value:T, expires:number|string|Date = this.defaultExpires):Observable<T>{
    let _expires:Date = this.sanitizeAndGenerateDateExpiry(expires);

    return this.localstorage.setItem(key, {
      expires: _expires,
      value: value
    }).map(val => val.value);
  }

  /**
   *
   * @param key
   * @returns {Observable<null>}
   */
  expire(key:string):Observable<null>{
    return this.localstorage.removeItem(key);
  }

  /**
   *
   * @param expires
   * @returns {Date}
   */
  private sanitizeAndGenerateDateExpiry(expires:string|number|Date):Date{
    let expiryDate:Date = this.expiryToDate(expires);

    //Dont allow expiry dates in the past
    if(expiryDate.getTime() <= Date.now()){
      return new Date(Date.now() + this.defaultExpires);
    }

    return expiryDate;
  }

  /**
   *
   * @param expires
   * @returns {Date}
   */
  private expiryToDate(expires:number|string|Date):Date{
    if(isNumber(expires)){
      return new Date(Date.now() + Math.abs(expires)*1000);
    }
    if(isString(expires)){
      return new Date(expires);
    }
    if(isDate(expires)){
      return expires;
    }

    return new Date();
  }
}

/**
 * Cache storage record interface
 */
interface CacheStorageRecord {
  expires: Date,
  value: any
}

local-storage.service.ts

import * as localforage from 'localforage';
import {Injectable} from "@angular/core";
import {Observable} from "rxjs/Observable";

@Injectable()
export class LocalStorageService {

  /**
   *
   * @param key
   * @param value
   * @returns {any}
   */
  public setItem<T>(key:string, value:T):Observable<T>{
    return Observable.fromPromise(localforage.setItem(key, value))
  }

  /**
   *
   * @param key
   * @returns {any}
   */
  public getItem<T>(key:string):Observable<T>{
    return Observable.fromPromise(localforage.getItem(key))
  }

  /**
   *
   * @param key
   * @returns {any}
   */
  public removeItem(key:string):Observable<void>{
    return Observable.fromPromise(localforage.removeItem(key))
  }
}