var file_extension = ‘txt’; if (file_extension === ‘txt’) alert(‘plain text’); else if (file_extension === ‘md’) alert(‘markdown’); var file_extension = ‘txt’; if (file_extension === ‘txt’) alert(‘plain text’); if (file_extension === ‘md’) alert(‘markdown’); The second example is wrong: there is no need to use the second if. I would say the use of the second if is ..