Changing the last character of a file

I want to continuously write json objects to a file. To be able to read it, I need to wrap them into an array. I don't want to read the whole file, for simple appending. So what I' doing now:

comma := []byte(", ")
    file, err := os.OpenFile(erp.TransactionsPath, os.O_WRONLY|os.O_APPEND|os.O_CREATE, 0666)
    if err != nil {
        return err
    }
    transaction, err := json.Marshal(t)
    if err != nil {
        return err
    }
    transaction = append(transaction, comma...)
    file.Write(transaction)

But with this implementation I will need to add []scopes by hand(or via some script) before reading. How can I add an object before closing scope on each writing?

#json #go