Skip to content

How to solve the `TypeError: Attempted to assign to readonly property` error

I was doing something in my Next.js codebase when I ran into this problem:

TypeError: Attempted to assign to readonly property

Weird! After a bit of debugging I found the problem. I has nothing to do with Next.js, it can happen in any JavaScript codebase.

I had a column in my database where I stored data as JSON.

In my code I was updating this JSON object, using the dot syntax (like data.name = 'Flavio') but I forgot to call JSON.parse() before doing so.

data was not an object, but a string!

Strings are immutable in JavaScript. We can’t update them once defined. Hence the error. The solution was to, obviously, call JSON.parse() before updating the JSON object.


→ Here's my latest YouTube video

→ Get my JavaScript Beginner's Handbook

→ I wrote 17 books to help you become a better developer, download them all at $0 cost by joining my newsletter

JOIN MY CODING BOOTCAMP, an amazing cohort course that will be a huge step up in your coding career - covering React, Next.js - next edition February 2025

Bootcamp 2025

Join the waiting list