Simple Javascript Modules - Local Storage Module

#todayilearned #javascript #webdev #beginners

Working as a web developer I work on the client side dealing with lots of continuous data from the server. All though it is a good practice to not store data on the client side, there will be many cases where we want to store some information on the client for faster access. A good use case would be the app theme theme: "light" or theme: "dark".

So that is where we can use Local Storage of the Browser using the window.localStorage API. It is a free storage space available for almost all major browsers and we can save a considerable amount of safe data for quick use in our application. It can be found on almost all major browsers. You can view, modify or delete the local storage data under the Application Tab in your browser followed by Local Storage and clicking your domain.

As you can see here in the code below, I have added all the things that we mostly do with local storage. Usually all these functions are separately written in a utility module. But here I have kept it separately so that in any case of change, we just replace the functions inside the module and our application code remains untouched. The whole point of writing these functions together as a module is to keep the code clean, crisp and understandable.

// A localStorage object for performing crud operations on window.localStorage with ease

const Storage = {
  // check if local storage is supported on browser
  exists: (window.localStorage !== undefined),
  // get the local storage data of the key
  get(key) {
    if (!this.exists) return;
    const data = window.localStorage.getItem(key);
    if (!data) return;
    return JSON.parse(data);
  },
  // add a new data to given key in local storage
  add(key, data) {
    if (!this.exists) return;
    window.localStorage.setItem(key, JSON.stringify(data));
  },
  // remove the data by the given key from local storage
  remove(key) {
    if (!this.exists) return;
    window.localStorage.removeItem(key);
  },
  // wipe entire local storage of current domain
  wipe() {
    if (!this.exists) return;
    window.localStorage.clear();
  }
}

export default Storage;

I have written a total of 5 properties on my module here which will use the window.localStorage API and are as follows.

1.exists - This contains a boolean value that checks if the current browser supports local storage or not.

2.get(key) - This function is used to get the data that is associated with the key that is sent in the parameters. For example if get("name") will get you the data that is saved under the name key in local storage. This function calls the window function called localStorage.getItem() which takes 1 parameter "key".

3.add(key, data) - This function is used to add new data to the local storage. The key attribute specifies to which key the data should be added to and the data attribute contains the data to be added. This function calls the window function called localStorage.setItem() which takes 2 parameters "key" and "data". It could be anything like a string, number, array, object etc.

For example running this =>

Storage.add("myKey", { name: "Sooraj" })

Will add this under the key called "myKey"

4.remove(key) - This function is used to remove the data associated with the key that is sent in the parameters. This function calls the window function called localStorage.removeItem() which takes 1 parameter "key". If remove("myKey") is called the data that was added before would be cleared from the storage.

5.wipe() - This is a method I would not use that many times. This function calls the window function called localStorage.clear() which takes no parameters. This function clears all the local storage data associated with the client on that domain.

Well, this is my Storage module. Hope you guys found it useful. Go ahead and try it out...

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Top comments (14)

Mihail Malo • Jun 29 '21 • Edited

I'm sorry, but this code makes no sense and is frankly confusing and dangerous.
It doesn't really add any convenience over just using window.localStorage directly, besides the JSON conversion.
This can be achieved with just

export class JSONStorage {
  constructor(storage) {
    if (!storage || typeof storage !== "object")
      throw new Error(`Expected a Storage object, got ${storage}`)
    this.storage = storage
  }
  set(k, v) {
    const str = JSON.stringify(v)
    if (typeof str === "undefined")
      return this.storage.removeItem(k)
    this.storage.setItem(k, v)
  }
  get(k) {
    const str = this.storage.getItem(k)
    // we return `undefined` instead of `null`, so that we can differentiate between the cases when the key doesn't exist vs when the value is a JSON-serialized `"null"`
    if (str === null) return
    return JSON.parse(str)
  }
  remove(k) {
    this.storage.removeItem(k)
  }
}

you can try it as follows

const a = new JSONStorage(window.localStorage)
const b = new JSONStorage(window.sessionStorage)
a.set("1", 1)
b.set("1", 2)
// notice that keys are coerced to string automatically, and we get back deserialized numbers, not strings
a.get(1) // 1
b.get(1) // 2

Instead, the provided code:

checks if localStorage is supported on every action, instead of just once.
calls .getItem(key) twice, which might be the most expensive operation here, short of setItem
similarly, calls getItem before removeItem
since !"" is true, empty values (if created outside this interface) will never be removed!
uses a mutable field data on the singleton object instead of a local variable. This data never needs to survive beyond a function call, and is being reused by all accesses to Storage.
constructs a {key, data} object with the function's arguments, which will never be used and doesn't offer any ergonomic benefit.
declares a function to just set null to this.data.
on most error conditions (such as passing a number as key, because numbers don't have .length), randomly returns the contents of this.data, which will always be null because every method, even remove that doesn't use it first calls clearData. This is confusing, since the standard getItem returns null for missing key, and "null" is a valid JSON string.
makes an object instance named Storage to then just export it as default, instead of exporting the individual functions, avoiding a layer of indentation.

PS: ~~Keep in mind, my code still doesn't prevent you from calling, say, b.set() which would create a undefined => undefined entry, which would cause a JSON SyntaxError when calling b.get(),~~ nor does it exhaustively check for the storage object passed to the constructor having the necessary methods.

Edit: The former is quite serious, so I fixed it now. This now removes the key, so setting a value of undefined and then getting it will, in fact, return undefined

Sooraj (PS) • Jun 29 '21

Thank you for the informative points :)

John Smith • Jun 27 '21

Comment hidden by post author

Sooraj (PS) • Jun 27 '21

Thank you John :)

Sooraj (PS) • Jun 27 '21 • Edited

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Preethi Vuchuru27916 • Jun 26 '21

As a newbie to the concept of Local storage in JavaScript, your comments on the code and explanation made it simple for me to understand. Will be using this to create something in Javascript.