Mastering JavaScript

JavaScript Primer
• Use the Boolean() function; this is an ordinary function that returns a
primitive Boolean:
var fBooleanTrue = Boolean(true);
var fBooleanFalse = Boolean(false);
Both these methods return expected truthy or falsy values. However, if you create a
Boolean object using the new operator, things can go really wrong.
Essentially, when you use the new operator and the Boolean(value) constructor,
you don't get a primitive true or false in return, you get an object instead—and
unfortunately, JavaScript considers an object as truthy:
var oBooleanTrue = new Boolean(true);
var oBooleanFalse = new Boolean(false);
console.log(oBooleanTrue); //true
console.log(typeof oBooleanTrue); //object
if(oBooleanFalse){
console.log("I am seriously truthy, don't believe me");
}
>"I am seriously truthy, don't believe me"
if(oBooleanTrue){
console.log("I am also truthy, see ?");
}
>"I am also truthy, see ?"
//Use valueOf() to extract real value within the Boolean object
if(oBooleanFalse.valueOf()){
console.log("With valueOf, I am false");
}else{
console.log("Without valueOf, I am still truthy");
}
>"Without valueOf, I am still truthy"
So, the smart thing to do is to always avoid Boolean constructors to create a new
Boolean object. It breaks the fundamental contract of Boolean logic and you should
stay away from such difficult-to-debug buggy code.
[ 14 ]
www.it-ebooks.info