Question

If a value must be defined to true or false, based off the check off another value, are there any gotchas in doing :

!!someValue

Just a quick run against chrome’s engine gives:

boolCheck=function(someValue){
    return !! someValue;
}

console.log(boolCheck());           //false
console.log(boolCheck(undefined));  //false
console.log(boolCheck(null));       //false
console.log(boolCheck(1));          //true
console.log(boolCheck({va:1}));     //true
console.log(boolCheck("someTS"));   //true
console.log(boolCheck(boolCheck));  //true    

which SEEMS to work….

Answer 1

The ! operator always returns either true or false, so assuming evaluation of x completes normally, !!x is a boolean value that is the equivalent of Boolean(x) where Boolean is the EcmaScript builtin function.

http://es5.github.com/#x11.4.9 says

11.4.9 Logical NOT Operator ( ! )

The production UnaryExpression : ! UnaryExpression is evaluated as follows:

1. Let expr be the result of evaluating UnaryExpression.
2. Let oldValue be ToBoolean(GetValue(expr)).
3. If oldValue is true, return false.
4. Return true.

http://es5.github.com/#x9.2 explains how ToBoolean works

9.2 ToBoolean

The abstract operation ToBoolean converts its argument to a value of type Boolean according to Table 11:

Table 11 — ToBoolean Conversions

Argument Type    Result 
Undefined        false
Null             false
Boolean          The result equals the input argument (no conversion).
Number           The result is false if the argument is +0, −0, or NaN; otherwise the result is true.
String           The result is false if the argument is the empty String (its length is zero); otherwise the result is true.
Object           true

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The above table explains most of your examples, but it may not be obvious why

 console.log(boolCheck());

is false. When you call a function with fewer actual arguments than formal parameters, the extra parameters have the value undefined and, as the table above shows !!undefined is false.

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are there any gotchas in doing : !!someValue?

The intent is less clear than Boolean(someValue), but it will work consistently across platforms and most experienced Javascript devs will recognize it.