I'm having a problem in Safari desktop / mobile and Internet Explorer. The error reads:

In Safari: `TypeError: Attempted to assign to readonly property.`

IE Edge: `Assignment to read-only properties is not allowed in strict mode`

It is happening in a function that loops through all textareas on the page and sets a specific css class on each textarea according to the character count (which changes the font-size).

My questions are:

* Why is this error occurring when I'm just applying a css class?
* What can I do to prevent it?

I'm using Laravel 5.4 and VueJS 2.1.10.

Here is [a codepen link](https://codepen.io/jumprope-design/pen/vmQvpw?editors=1010) where I have replicated the problem. (You can see it works in Chrome and FireFox).

I will really appreciate any help. Thanks