Double negation (!!) in javascript - what is the purpose?
JavascriptJavascript Problem Overview
> Possible Duplicate:
> What is the !! (not not) operator in JavaScript?
I have encountered this piece of code
function printStackTrace(options) {
options = options || {guess: true};
var ex = options.e || null, guess = !!options.guess;
var p = new printStackTrace.implementation(), result = p.run(ex);
return (guess) ? p.guessAnonymousFunctions(result) : result;
}
And couldn't help to wonder why the double negation? And is there an alternative way to achieve the same effect?
(code is from https://github.com/eriwen/javascript-stacktrace/blob/master/stacktrace.js)
Javascript Solutions
Solution 1 - Javascript
It casts to boolean. The first !
negates it once, converting values like so:
undefined
totrue
null
totrue
+0
totrue
-0
totrue
''
totrue
NaN
totrue
false
totrue
- All other expressions to
false
Then the other !
negates it again. A concise cast to boolean, exactly equivalent to ToBoolean simply because !
is defined as its negation. It’s unnecessary here, though, because it’s only used as the condition of the conditional operator, which will determine truthiness in the same way.
Solution 2 - Javascript
var x = "somevalue"
var isNotEmpty = !!x.length;
Let's break it to pieces:
x.length // 9
!x.length // false
!!x.length // true
So it's used convert a "truethy" "falsy" value to a boolean.
The following values are equivalent to false in conditional statements:
- false
- null
- undefined
- The empty string
""
(\''
) - The number 0
- The number NaN
All other values are equivalent to true.
Solution 3 - Javascript
Double-negation turns a "truthy" or "falsy" value into a boolean value, true
or false
.
Most are familiar with using truthiness as a test:
if (options.guess) {
// runs if options.guess is truthy,
}
But that does not necessarily mean:
options.guess===true // could be, could be not
If you need to force a "truthy" value to a true boolean value, !!
is a convenient way to do that:
!!options.guess===true // always true if options.guess is truthy