responseText.lastIndexOf('\n', responseText.length - 2)

This code is using the `lastIndexOf()` method to find the index of the last occurrence of the newline character (`\n`) in a string called `responseText`. The second argument passed to `lastIndexOf()` is `responseText.length - 2`, which means it will start searching for the newline character from the second-to-last character in the string. This is because the last character in the string is likely to be a newline character, and we want to find the one before that. Overall, this code is likely being used to extract the last line of text from `responseText`, which may contain multiple lines of text separated by newline characters.