The unary + operator casts the instance to a Number in the same way that calling the Number() function will cast a variable to a Number.

If you'd like to override how your specific instance is cast, you need to override the valueOf property for that instance:

var a = {
    valueOf: function () {
        return 5;
    }
};
console.log(a); //Object { valueOf: function () {...} }
console.log(+a); //5

From the ES5 standard:

11.4.6 Unary + Operator # Ⓣ Ⓡ Ⓖ

The unary + operator converts its operand to Number type.

The production UnaryExpression : + UnaryExpression is evaluated as follows:

1. Let expr be the result of evaluating UnaryExpression.
2. Return ToNumber(GetValue(expr)).

ToNumber on an Object will then result in a ToPrimitive with hint Number. ToPrimitive will then call the [[DefaultValue]] internal method which states:

When the [[DefaultValue]] internal method of O is called with hint Number, the following steps are taken:

1. Let valueOf be the result of calling the [[Get]] internal method of object O with argument "valueOf".
2. If IsCallable(valueOf) is true then,
   1. Let val be the result of calling the [[Call]] internal method of valueOf, with O as the this value and an empty argument list.
   2. If val is a primitive value, return val.
3. Let toString be the result of calling the [[Get]] internal method of object O with argument "toString".
4. If IsCallable(toString) is true then,
   1. Let str be the result of calling the [[Call]] internal method of toString, with O as the this value and an empty argument list.
   2. If str is a primitive value, return str.
5. Throw a TypeError exception.