Is any solution to do localstorage setItem in asynchronous way in javascript

Using ionic app validating token based auth, Storing the token in localstorage it is taking time to store in between it is moving to next state is any solution to do asynchronous way to set value in localstorage

window.localStorage.setItem('ChemistloggedInUser', JSON.stringify(data))

Solution

localStorage is a synchronous API. You could defer the setItem method execution with the Promise object, giving them an asynchronous behaviour:

const asyncLocalStorage = {
    setItem(key, value) {
        return Promise.resolve().then(function () {
            localStorage.setItem(key, value);
        });
    },
    getItem(key) {
        return Promise.resolve().then(function () {
            return localStorage.getItem(key);
        });
    }
};

// Demo
const data = Date.now() % 1000;
asyncLocalStorage.setItem('mykey', data).then(function () {
    return asyncLocalStorage.getItem('mykey');
}).then(function (value) {
    console.log('Value has been set to:', value);
});
console.log('waiting for value to become ' + data + 
            '. Current value: ', localStorage.getItem('mykey'));

(Instead of Promise.resolve().then you can also use queueMicrotask. It achieves the same, but without creating/returning a promise, so you cannot chain a then call then).

With the newer async/await syntax, this asyncLocalStorage can be written as:

const asyncLocalStorage = {
    async setItem(key, value) {
        await null;
        return localStorage.setItem(key, value);
    },
    async getItem(key) {
        await null;
        return localStorage.getItem(key);
    }
};

Note about "asynchronous"

Be aware that, although the above let's you continue with other code immediately, once that code has been executed, the job of accessing local storage will kick in and will use the same thread. So it is not like it runs in the background, in parallel with your own JS code. It just delays the job until after the call stack is empty. It also does not process other events from the browser context until it also finishes that job. So, for instance, it will still block the GUI.

If you need the access to happen in parallel, then you're out of luck with localStorage. For instance, that API is not available in Web Workers, which would otherwise have been a possible way out.

You could look into the IndexedDB API as an alternative. But:

It is much more complicated to work with
Although it has an asynchronous interface, several browser implementations still block the DOM (so the above comments apply)
IndexedDB can be used by a Web Worker, which gives better parallelism, but makes it even more complex to implement.