My code journey

JSON.parse() requires a string, so we have to make sure that it gets one

October 09, 2020

I was trying to create a language switch that allows the user to change the language in which the site is displayed. I therefore stored the current language in the localStorage.

At some other point I had to read the current language from localStorage and I tried doing it first in the following way:

const currentLanguage = localStorage.getItem('savedValues')

However, as I quickly discovered, it’s not possible to return the name of the current language by writing

The reason is that values stored in localStorage are stored as strings and it’s therefore not possible to reference its values with a dot notation (the latter required an object).

In order to construct an object from a JSON string, we can use JSON.parse().

But writing JSON.parse(localStorage.getItem('savedValues')) produces an error:

Argument of type ‘string | null’ is not assignable to parameter of type ‘string’. Type ‘null’ is not assignable to type ‘string’.

The reason is that localStorage.getItem() can return either a string or null. JSON.parse() requires a string, so I had therefore to test the result of localStorage.getItem() before I’m using it:

const currentLanguage = JSON.parse(localStorage.getItem('savedValues') || '{}')

If localStorage.getItem() returns null we return JSON.parse() curly braces {} which will be interpreted by the latter as an empty object.

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My name is Pascal Witzig, and I'm a Full Stack Web Dev working with Follow me on Twitter

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