what is the meaning of +!! in javascript?

Question

I came across this line of code recently, and want to understand what it means and does, as my javascript-foo is not that hot :

if ((+!!config.template) + (+!!config.templateUrl) !== 1) {}

from what I can gather, it is checking to see if either option is set (so either template, or templateUrl must be set, not both or none)

so, if config.template was set,

+config.template would not work (template is not a number)
!config.template would return false (-1)
!!config.template would return true (0)
+!!config.template would therefore return 1

if config.template was not set,

!config.template would return true (0)
!!config.template would return false (-1)
+!!config.template would therefore return 0

if then you were to apply the same to config.templateUrl , you would end up with 1 if set, 0 if not

So the final test is to see if the sum of config.template and config.templateUrl is 1 (ie one or the other is set)

Is this valid reasoning ?

Almost right, except that +(true) === 1 and +(false) === 0 — Matteo Tassinari
– Matteo Tassinari, Commented May 29, 2015 at 9:16
Actually the above expression is the equivalent to if (!config.template === !config.templateUrl) {}. It looks much simpler that way. — kreig
– kreig, Commented May 29, 2015 at 9:24

RoryGilchrist · Accepted Answer · 2015-05-29 09:53:50Z

3

The bool value is being cast to a Number by prepending it with +.

!! in the code above is checking for existence of the property template on config. If there is no template found !! would usually return false, however by prepending +, it returns 0. Both +!! statements return numbers, which when they're added together will either be 0 or 1.

The final check will return true if both or none were set / true (!== 1)

edited May 29, 2015 at 9:53

answered May 29, 2015 at 9:19

RoryGilchrist

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5 Comments

jmls Over a year ago

@RoryGilchrist : why would the code fail if both were set ? Yes, the expression would be 2, but the test checks for !== 1 , therefore it would execute the code in {} (which was the intention of the author - a message saying "you must specify one or the other) or am I being thick ?

RoryGilchrist Over a year ago

You're right, I misread. So basically in the code above, it's want's either none or both to be set, but not just one.

Armfoot Over a year ago

@RoryGilchrist exactly so: "which when they're added together will be 0, 1 or 2".

RoryGilchrist Over a year ago

@Armfoot Other way round amigo, it would be false if it were 1, but not 2. 1 !== 1 is fasle because 1 == 1, but 2 !== 1 is true because 2 is not 1.

Armfoot Over a year ago

@RoryGilchrist yeah xD nevermind, I edited that... I was thinking === from the question's comment.

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