How to take advantage of Local Storage in your React projects

Originally published by Ryan J. Yost on April 15th 2018

@ryanjyostRyan J. Yost

And why you ought to.

Local Storage is a Web API native to modern web browsers. It allows websites/apps to store data (simple and limited) in the browser, making that data available in future browser sessions.

Before diving into the tutorial, it may be unclear why you’d want to even use Local Storage in your React apps.

There’s plenty of reasons and use cases, more than I can imagine, but here’s a few I’ve discovered.

• A simple, fake backend for your frontend React projects — It’s often nice to add the appearance of a backend/database to your frontend portfolio projects. The extra functionality will take your app to the next level, improve the user experience and impress potential employers.
• Experiment with different states while developing — When working on an app, it’s often useful or necessary for the app to have a certain state to be able to work on particular styling and functionality, (e.g. styling a list of items and removing items requires items). Rather than recreating an app’s state on every refresh, Local Storage can persist that state, making development much more efficient and enjoyable.
• Saving form data across sessions — what do people hate more than filling out a form? Filling out a form twice!

Getting Started

Create a new React project using create-react-app.

npx create-react-app local-storage

cd into the new directory and fire up the app. Install yarn if you haven’t already.

yarn start

Update your App.js with the code below. Here, we’re setting up a simple to-do list app. Absolutely nothing fancy, but it’ll do the trick for playing with localStorage.

After copying over this code, you should be able to add to-do items to the list and remove them.

Start saving things to localStorage

Saving the value of our newItem input to localStorage is a piece of cake.

In the updateInput() method, we’ll invoke the localStorage.setItem() method, which takes two arguments:

• key: string — the name of the localStorage item
• value: string — the value you want to save for the given localStorage key. Note: Even arrays and objects need to be saved as strings. More on that in a bit.

Here’s our new updateInput() method.

As you can see, it’s not much different from updating React state.

Open up the Web Developer tools in your browser of choice, find the section for Web Storage (“Application” tab in Chrome), select the current domain of localhost:3000 and watch the value for the newItem key stay in sync with your app’s input.

Now, let’s save the list of to-do items

When adding an item, we save the new, updated list to localStorage and reset the newItem input to a blank string.

No surprises here, except for the use of JSON.stringify(). This method converts a JavaScript value into a JSON string.

Because localStorage can only store strings, arrays and objects need to be passed into JSON.stringify() before being passed to setItem().

Before moving on, we’ll also want to update the list in localStorage when deleting an item.

Ok, we’re saving. But watch what happens when you refresh the page…

…the App reverts back to its initial state! We aren’t using the stored items yet, just saving them in the background. Not terribly helpful…

In order to persist the app’s state even after refreshing the page, we need to hydrate the App‘s state with the values in localStorage, with help from a couple new methods:

• localStorage.getItem() — takes a storage key and returns the value saved under that key.
• JSON.parse() — converts a JSON string into a JavaScript value. You need this to correctly retrieve objects and arrays that were saved as strings to localStorage.

The below method hydrates the app’s state with the values saved to localStorage. Add this new method to your App component.

It makes sense to hydrate state when the page loads, i.e. early on in the component lifecycle. So let’s invoke this function in componentDidMount().

Once you’ve added the above code to your App component, refreshing the page no longer resets the app, but keeps it in sync with localStorage!

Continuously saving is unnecessary — there’s a better way.

While our app works as-is by saving React state to localStorage every time the user makes an update, we don’t really need to save so frequently.

Why? Because React keeps track of the state of the app throughout the user’s session — that’s what it’s for! Also, with more complex components and states, it’ll be quite cumbersome and repetitive to use localStorage.setItem() wherever the state is updated.

So rather than continuously keeping localStorage in-sync with React state, let’s simply save state to localStorage whenever the user ends their session, either by leaving the app (‘unmounting’ the component) or refreshing the page.

The new sequence of events/operations will be…

1. The user visits the app (localhost:3000 in our case)
2. The App component mounts and hydrates its state with any applicable localStorage values.
3. React will update state throughout the user’s session. localStorage won’t change.
4. When the user ends their session, save whatever the state is at that time to localStorage, making it available for hydrating in the next session.

Alright, here’s a new method for saving all of state to localStorage at once. Add it to your App component.

In order to save state to localStorage when the user leaves the app, we need to invoke the saveStateToLocalStorage method in componentWillUnmount.

CAVEAT — componentWillUnmount does not fire when the user refreshes or leaves the page, so we need to utilize the window.onbeforeunload event to be able to save to localStorage. Learn more here.

Here’s some updated code, where we add the event listener to componentDidMount as well as add what we need to componentWillUnmount.

We no longer need to setItem when updating React state, so you’ll want to remove those.

A lot has changed since the beginning of the tutorial, so here’s the App.js file at this point. *Nothing in the render() method has changed.*

And that’s it! You now have the tools to use Local Storage in your React projects.

React Simple Storage — an almost shameless plug

I found myself at work wanting to take advantage of Local Storage in tons of different components, so I created a component, react-simple-storage, that handles everything we’ve just implemented and much more. Keep following along to see how easy it is to use in our little to-do app.

1. Install it

yarn add react-simple-storage

2. Import it into App.js

3. Include it in your <App> component, like this…

That’s it! You don’t need all of the extra methods and event listeners from the tutorial, so the final App.js using react-simple-storage looks like this…

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