+ operator in Javascript

Question

So I've noticed that the result of

(new Date())

is a Date object; in this case

Date {Thu Dec 04 2014 22:43:07 GMT+0200 (SAST)}

but if I type

+(new Date())

I get an int value;

1417725787989

How is this done?

I have a function called 'Duration' which, when used like this:

new Duration(352510921)

returns an instance looking like this:

{ days:5, hours:3, mins:55, secs:10, ms:921 }

So how can I use the + operator to get the int value of the Duration instance?

var dur = new Duration(352510921);
console.log(+dur) // prints int value 352510921

http://stackoverflow.com/questions/221539/what-does-the-plus-sign-do-in-return-new-date — j08691, Dec 04 '14 at 20:53

score 5 · Accepted Answer · edited Jun 20 '20 at 09:12

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:

Let expr be the result of evaluating UnaryExpression.

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:

Let valueOf be the result of calling the [[Get]] internal method of object O with argument "valueOf".

If IsCallable(valueOf) is true then,

Let val be the result of calling the [[Call]] internal method of valueOf, with O as the this value and an empty argument list.

If val is a primitive value, return val.

Let toString be the result of calling the [[Get]] internal method of object O with argument "toString".

If IsCallable(toString) is true then,

Let str be the result of calling the [[Call]] internal method of toString, with O as the this value and an empty argument list.

If str is a primitive value, return str.

Throw a TypeError exception.

Thanks so much, that means I could add a 'toString' function property to my 'Duration' definition and by using: `'' + new Duration(352510921);` I could have it print out `5 days, 3 hours, 55 minutes, 10 seconds and 921 milliseconds` Thanks for the explanation! — Gavin van Gent, Dec 04 '14 at 21:20

score 3 · Answer 2 · answered Dec 04 '14 at 20:54

3

You can override valueOf.

> var foo = { days:5, hours:3, mins:55, secs:10, ms:921, valueOf: function() { return 1; }  }   
undefined
> -foo
-1

answered Dec 04 '14 at 20:54

Daniel A. White

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+ operator in Javascript

2 Answers2

11.4.6 Unary + Operator # Ⓣ Ⓡ Ⓖ