The time it takes our applications to show useful information for our users has a great impact on the user experience. That’s why I think it’s our responsibility as software developers to implement mechanisms that allow us to reduce this loading time as much as possible.

In this article, I’m gonna show you how to implement client-side caching with Angular. By the end of this post, you’ll be able to cache your http request like this:

return this._http.get<Product[]>({ url: 'https://example-api/products', cacheMins: 5 })

For this implementation, we’ll need:

  • A cache service: This service will be required for two main things:
    • Save data in the localstorage (with expiration)
    • Load data from the localstorage.
  • A custom http-client service: This service will use the angular HttpClient under the hood, but will also use the cache service mentioned above to get and save data from/to localstorage.

cache.service.ts

import { Injectable } from '@angular/core'

@Injectable()
export class CacheService {
	constructor() { }

	save(options: LocalStorageSaveOptions) {
		// Set default values for optionals
		options.expirationMins = options.expirationMins || 0

		// Set expiration date in miliseconds
		const expirationMS = options.expirationMins !== 0 ? options.expirationMins * 60 * 1000 : 0
		
		const record = {
			value: typeof options.data === 'string' ? options.data : JSON.stringify(options.data),
			expiration: expirationMS !== 0 ? new Date().getTime() + expirationMS : null,
			hasExpiration: expirationMS !== 0 ? true : false
		}
		localStorage.setItem(options.key, JSON.stringify(record))
	}

	load(key: string) {
		// Get cached data from localstorage
		const item = localStorage.getItem(key)
		if (item !== null) {
			const record = JSON.parse(item)
			const now = new Date().getTime()
			// Expired data will return null
			if (!record || (record.hasExpiration && record.expiration <= now)) {
				return null
			} else {
				return JSON.parse(record.value)
			}
		}
		return null
	}

	remove(key: string) {
		localStorage.removeItem(key)
	}

	cleanLocalStorage() {
		localStorage.clear()
	}
}

export class LocalStorageSaveOptions {
	key: string
	data: any
	expirationMins?: number
}

http-client.service.ts

import { Injectable } from '@angular/core'
import { HttpClient } from '@angular/common/http'
import { CacheService } from './cache.service'
import { Observable, of } from 'rxjs'
import { switchMap } from 'rxjs/operators'

export enum Verbs {
	GET = 'GET',
	PUT = 'PUT',
	POST = 'POST',
	DELETE = 'DELETE'
}

@Injectable()
export class HttpClientService {

	constructor(
		private http: HttpClient,
		private _cacheService: CacheService,
	) { }

	get<T>(options: HttpOptions): Observable<T> {
		return this.httpCall(Verbs.GET, options)
	}

	delete<T>(options: HttpOptions): Observable<T> {
		return this.httpCall(Verbs.DELETE, options)
	}

	post<T>(options: HttpOptions): Observable<T> {
		return this.httpCall(Verbs.POST, options)
	}

	put<T>(options: HttpOptions): Observable<T> {
		return this.httpCall(Verbs.PUT, options)
	}

	private httpCall<T>(verb: Verbs, options: HttpOptions): Observable<T> {

		// Setup default values
		options.body = options.body || null
		options.cacheMins = options.cacheMins || 0

		if (options.cacheMins > 0) {
			// Get data from cache
			const data = this._cacheService.load(options.url)
			// Return data from cache
			if (data !== null) {
				return of<T>(data)
			}
		}

		return this.http.request<T>(verb, options.url, {
			body: options.body
		})
			.pipe(
				switchMap(response => {
					if (options.cacheMins > 0) {
						// Data will be cached
						this._cacheService.save({
							key: options.url,
							data: response,
							expirationMins: options.cacheMins
						})
					}
					return of<T>(response)
				})
			)
	}
}

export class HttpOptions {
	url: string
	body?: any
	cacheMins?: number
}

Now, let’s say we have a product service we use to retrieve a list of products from our API. In this service we’ll use our recently created http-client service to make a request and save the data in the localstorage for 5 minutes:

// product.service.ts

import { Injectable } from '@angular/core'
import { HttpClientService } from './http-client.service'
import { Observable } from 'rxjs'

@Injectable()
export class ProductService {

	constructor(
		private _http: HttpClientService
	) { }

	getAll(): Observable<Product[]> {
		return this._http
			.get<Product[]>({ url: 'https://example-api/products', cacheMins: 5 })
	}
}

export class Product {
	name: string
	description: string
	price: number
	available: boolean
}

What do you think about this strategy? Are you using other techniques like http-interceptor? Let me know in the comments below

This post is also available on DEV.