The output will be the string '53'. When the + operator is used with a string and a number, JavaScript converts the number to a string and performs string concatenation. In this case, the number 3 is converted to the string '3', and then concatenated with the string '5', resulting in '53'. This is different from operations like '5' - 3, where the string '5' would be converted to the number 5, resulting in 2.