What does ! mean?

[李立峰]

What does !! in javascript mean?

I cannot find it in http://www.w3schools.com/js/js_operators.asp

 

And also, where can I look up these operators? You know, typing operators (like !!) in Google normally returns nothing..

 

Thanks!

[dsonesuk]

Google search 'double exclamation operator' instead

[Ingolme]

It's actually on this page in the section about logical operators: http://www.w3schools.com/js/js_comparisons.asp

 

If it helps to understand it better, !!something is the same as !(!something)

[justsomeguy]

To put it more simply, "!!" is not one operator, it is two operators being used at once.

[dsonesuk]

! on its own by default with boolean variable like if(!something) would check if value is NOT true (I.E false), adding second if(!!something) would reverse previous comparison to check value IS true, so as you add more exclamation marks it would give opposite comparison to previous, NOT true, IS true, NOT true and so on....

 

Where

if(!something) //not true

 

or

 

if(something) // is true

 

would suffice

[justsomeguy]

The main purpose is to cast something as a boolean. e.g.:

var obj = {prop: 'val'}; // this is an objectvar bool = !!obj; // this is a boolean, true if obj is not a "falsy" value