Determine if a Javascript Variable is a Number

This function determines if a javascript variable can be converted into a number.

function is_number(n) {
    return !isNaN(parseFloat(n)) && isFinite(n);
}

Example:

isNumber(5) returns true
isNumber("5") returns true
isNumber("5" + 5) returns true
isNumber("5 + 5") returns false
isNumber("5a") returns false
isNumber(5a) returns false