Declaring a boolean in JavaScript using just var

Question

If I declare a JavaScript boolean variable like this:

var IsLoggedIn;

And then initialize it with either true or 1

Answer 1

No it is not safe. You could later do var IsLoggedIn = "Foo"; and JavaScript will not throw an error.

It is possible to do

var IsLoggedIn = new Boolean(false);
var IsLoggedIn = new Boolean(true);

You can also pass the non boolean variable into the new Boolean() and it will make IsLoggedIn boolean.

var IsLoggedIn = new Boolean(0); // false
var IsLoggedIn = new Boolean(NaN); // false
var IsLoggedIn = new Boolean("Foo"); // true
var IsLoggedIn = new Boolean(1); // true

Answer 2

Variables in Javascript don't have a type. Non-zero, non-null, non-empty and true are "true". Zero, null, undefined, empty string and false are "false".

There's a Boolean type though, as are literals true and false.

Answer 3

You can use and test uninitialized variables at least for their 'definedness'. Like this:

var iAmNotDefined;
alert(!iAmNotDefined); //true
//or
alert(!!iAmNotDefined); //false

Furthermore, there are many possibilites: if you're not interested in exact types use the '==' operator (or ![variable] / !![variable]) for comparison (that is what Douglas Crockford calls 'truthy' or 'falsy' I think). In that case assigning true or 1 or '1' to the unitialized variable always returns true when asked. Otherwise [if you need type safe comparison] use '===' for comparison.

var thisMayBeTrue;

thisMayBeTrue = 1;
alert(thisMayBeTrue == true); //=> true
alert(!!thisMayBeTrue); //=> true
alert(thisMayBeTrue === true); //=> false

thisMayBeTrue = '1';
alert(thisMayBeTrue == true); //=> true 
alert(!!thisMayBeTrue); //=> true
alert(thisMayBeTrue === true); //=> false
// so, in this case, using == or !! '1' is implicitly 
// converted to 1 and 1 is implicitly converted to true)

thisMayBeTrue = true;
alert(thisMayBeTrue == true); //=> true
alert(!!thisMayBeTrue); //=> true
alert(thisMayBeTrue === true); //=> true

thisMayBeTrue = 'true';
alert(thisMayBeTrue == true); //=> false
alert(!!thisMayBeTrue); //=> true
alert(thisMayBeTrue === true); //=> false
// so, here's no implicit conversion of the string 'true'
// it's also a demonstration of the fact that the 
// ! or !! operator tests the 'definedness' of a variable.

PS: you can't test 'definedness' for nonexisting variables though. So:

alert(!!HelloWorld);

gives a reference Error ('HelloWorld is not defined')

(is there a better word for 'definedness'? Pardon my dutch anyway;~)

Answer 4

The variable will become what ever type you assign it. Initially it is undefined. If you assign it 'true' it will become a string, if you assign it true it will become a boolean, if you assign it 1 it will become a number. Subsequent assignments may change the type of the variable later.

Answer 5

As this very useful tutorial says:

var age = 0;

// bad
var hasAge = new Boolean(age);

// good
var hasAge = Boolean(age);

// good
var hasAge = !!age;

Answer 6

Types are dependent to your initialization:

var IsLoggedIn1 = "true"; //string
var IsLoggedIn2 = 1; //integer
var IsLoggedIn3 = true; //bool

But take a look at this example:

var IsLoggedIn1 = "true"; //string
IsLoggedIn1 = true; //now your variable is a boolean

Your variables' type depends on the assigned value in JavaScript.