While looking at the jslint code conventions I saw this line:
total = subtotal + (+myInput.value);
What is the purpose of the second '+'?
views:
637answers:
5
+30
A:
The unary plus is there for completeness, compared with the familiar unary minus (-x). However it has the side effect, relied upon here, of casting myInput.value into a Number, if it's something else such as a String:
alert(1+'2'); // 12
alert(1+(+'2')); // 3
bobince
2009-03-04 16:33:59
Thanks, I didn't know this notation. I assume this is basically shorthand for parseInt()?
Luke Dennis
2009-03-04 16:37:20
They're not quite the same, Luke, see here for some information: http://alexle.net/archives/290
Chad Birch
2009-03-04 16:39:08
Plus of course it would be closer to ‘parseFloat’. +-casting supports full floating point number literals, including weirdnesses like +'Infinity'.
bobince
2009-03-04 16:52:28
actually, the thing `(+foo)` is closest to is the explicit cast `Number(foo)` - in fact, they should yield identical results
Christoph
2009-03-04 18:44:09
Is there any reason why this should be preferred over parseInt?
toast
2009-03-04 19:27:19
@toast: they do different things: `Number()` and `+` will also convert floats and return `NaN` on invalid input, whereas `parseInt()` will just stop parsing on the first non-numeric character, ie `parseInt('42foo')` is `42`, whereas `(+'42foo')` is `NaN`
Christoph
2009-03-04 19:31:03
@Christoph: That's handy. I can see where they each could be used and why one would prefer a certain way for different conditions. Thanks
toast
2009-03-04 22:34:47
which is better in terms of execution time?
kunjaan
2009-04-14 16:16:06
In Firefox/Chrome, parseInt comes out 25% faster. In IE/Safari, it takes four times as long. Opera, twice as long. So there's no one conclusion you can draw. In any case, the amount of time it takes is minuscule; choosing one over the other on that ground would be totally a premature optimisation.
bobince
2009-04-14 16:56:39
@Luke Dennis: Want a shortcut for ‘parseInt(x, 10)’? Use ‘~~x’.
Mathias Bynens
2009-11-15 20:03:33
@Mathias: that's a 32-bit bitwise op, so it will give different results to `parseInt` for numbers out of range of a 32-bit signed int. eg. `~~'2147483648'`->`-2147483648`
bobince
2010-01-04 23:30:24
+8
A:
That's called the "unary + operator", it can be used as a quick way to force a variable to be converted to a number, so that it can be used in a math operation.
Chad Birch
2009-03-04 16:34:29
+2
A:
The + is to typecast it to a number as others have said. It's needed there because form inputs are always string values, and adding a string to another variable concatenates them together into a new string, even if the string looks like a number.
Ant P.
2009-03-04 16:37:25
+1
A:
The unary plus operator is, arithmetically speaking, a noop. But like all other purely arithmetic operators, it will convert its argument to JavaScript's number type and can therefore be used as a shorthand for an explicit cast.
Explicit type casting in JavaScript is done via calling the appropriate constructor functions without using the new operator.
For example,
Number(foo)
will convert foo to a primitive of type number, whereas new Number(foo) would additionally create a wrapper object for the primitive and is therefore equivalent to
Object(Number(foo))
Similar to this use of + is the use of !! to convert to boolean type.
Christoph
2009-03-04 19:00:13