“you could just write (fileName, query) => fileName.endsWith(query). Since the parameters are each used only once in the body of the function (i.e., the first parameter, fileName, is used first in the body, and the second parameter, query, is used second), you can use the placeholder syntax: _.endsWith(_). The first underscore is a placeholder for the first parameter, the file name, and the second underscore a placeholder for the second parameter, the query string.”