Javascript: How to write records into a file line by line separated by a comma but without a trailing comma at the end

I’ve got a file of records (one per line):

Record1
Record2
Record3

I’m using the fs and the readline modules for that:

        const rs = fs.createReadStream('input.txt');
        const ws = fs.createWriteStream('output.json', { encoding: "utf8" });
        const rl = readline.createInterface({
            input: rs,
            crlfDelay: Infinity,
        });
        ws.write('[');
        for await (const line of rl) {
            ws.write(`${line},n`);
        }
        ws.end(']');

But this loop will also toss in a trailing comma at the end of the file which is bad cos trailing commas are not allowed in JSON.

The problem is that I don’t want to read the whole file into memory and I don’t know the number of records in advance.

Edit:

This is a simple example, but in the real code, the record gets transformed to a json record, but it’s irrelevant.