I have some issues using the replace method for javascript strings :

I created a class that contains this method :

    setDataAt(pos, value) {
        this.#data = this.#data.substring(0, pos)+value+this.#data.substring(pos+1);
    }

but when I use it, the data is truncated as there wasn't any +this.#data.substring(pos+1);...
Per example, if my object obj has as #data attribute "Hello world", after making obj.setDataAt(5, '.'), the #data attribute will be "Hello." and not "Hello.world"

EDIT :

My issue doesn't seems to appair when I use it directly (below is a little part of my original code, all that is used into ma issue, and it totally work). But when I use it from my web page, it gives issues :

How to fix that? Thanks for all your answers

Another Edit :

The problem is that in my website page, the pos var isn't an integer, but a string. I have to replace all pos by parseInt(pos) to make it work correctly.

Thanks a lot @Nick_Paterson!