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Javascript – Data Persistence

How to save, update, and display dynamic data.

What is data persistence?

Data persistence describes storing/using data beyond the life of a program (or web page). For example:

  1. Add text to file, save it to your computer, and you can view it again later.
  2. A mobile app (e.g. Instagram) remembers you have already logged in.
  3. Use a website (e.g. Wordpress) to post text on the internet for others to see.

The technologies may change, but the concept does not—the software allows information to persist as part of its function, security, or customization.

Methods

In most examples, these software actually saves data in multiple locations:

Software Data on PC Data in client Data on network
TextEdit Text files to hard drive Settings in ~/Library/Preferences/ Files in the cloud
Instagram mobile app Edited photos in your library Stores preferences and user authentication Text in database, images on server via API
Google Docs Saves files for offline use Stores authentication (cookies) in browser Text in database, images on server

Client-side storage

Javascript in a browser can persist data in a browser in many ways:

  1. Cookies - A small piece of data stored in the browser.
  2. Web-based storage APIs (localStorage and sessionStorage)
  3. Cache - web browsers store copies of files locally in a cache so websites load faster.

Node.js (run-time Javascript) can write data directly to the hard drive.

Cookies

  • Cookies can store any string of text (user preferences, inputs, or authentication information) to customize a web experience.
  • A user can set their web browser to accept, reject, or delete cookies.
  • Cookies can be set and modified at the server level using the Set-Cookie HTTP header, or with JavaScript using document.cookie.

👉 Run these and look in the DevTools > Application. See MDN for more on cookies

document.cookie = "name=jane";
document.cookie = "message=hello, world!";
console.log(document.cookie);
//-> "name=jane; message=hello, world!"

LocalStorage

The Web Storage API allows you to store simple name/value pairs of data and retrieve these values when needed.

👉 Run these and look in the DevTools > Application

localStorage.setItem('location','Davidson College');
let location = localStorage.getItem("location");
console.log(location) //-> "Davidson College"

See this codepen to store an array or object with JSON.stringify()
See MDN for more on Client-side storage

Other client-side storage options

When you create an app that runs on its own (outside of a web browser) you have many additional options for storing data in the client.

  • For example, a mobile app has access to native features on the phone or device

Persisting data online

To save data for the world to see you need to run code on a server* like PHP or Node:

*Github Pages cannot run server-side scripts