onResponse changing values in selected attributes - doesn’t work

I am a tester with no programming skills. I use Maplocal or breakpoints on a daily basis. I noticed the scripts tab and wondered what it could be useful to me.
I am thinking about the case when I want to modify one value in the response without having to manually set up a breakpoint (situation when I need to have fresh /unique data in the response, but I need to change something specific)
As I do not know much about programming I asked the gpt chat to create such a script, unfortunately, despite many attempts/changes/analysis of errors, the data is not changed in the final response.
So, there is a simple response body (Json) with 3 attributes {"messages":10,"id":115,"card":0} , the goal is to change the response body for only messages value.

This is what i get from chatgpt:

function onResponse(context) {
    if (context.response && context.response.body) {
        let body = context.response.body;
        try {
            let json = JSON.parse(body);
            json.messages = 1;
            body = JSON.stringify(json);
            context.response.body = body;
            console.log("Modified response body:", body);
        } catch (e) {
            console.error("Error parsing JSON:", e);
        }
    } else {
        console.log("Response body is empty or undefined.");
    }
    return context.response;
}

and this is na answer :
Response body is empty or undefined.