The expression +'42' evaluates to the number 42. The unary plus operator (+) attempts to convert its operand to a number. When applied to a string that represents a valid number, it converts the string to the corresponding number. This is a concise way to convert strings to numbers, similar to using Number('42'). If the string can't be converted to a valid number (e.g., +'hello'), the result would be NaN (Not a Number).