JavaScript style: don't use wrapper objects for primitive types

Question

In the Google JavaScript style guide, it says not to use wrapper objects for primitive types. It says it's "dangerous" to do so. To prove its point, it uses the example:

var x = new Boolean(false);
if (x) {
  alert('hi');  // Shows 'hi'.
}

OK, I give up. Why is the if code being executed here?

Dumbwaiter

Because Boolean(false) === !(new Boolean(false)). And Boolean(false) === new Boolean(false).valueOf(). – Juanajuanita

@katspaugh: that makes it much clearer -- thanks. – Dumbwaiter

Answer 1

Because every variable that is typeof Object is truthy and wrappers are objects.

Misjudge

So basically you're saying that x is not being assigned the value false as I thought it was. It's being assigned an "object", which evaluates to true. – Dumbwaiter

Yes, an object that wraps the value, but as it is an object, it is truthy. – Misjudge

typeof null -> "object", !!null -> false. Correct me if I'm wrong, but I believe nulls are falsy objects and a rather important exception to the above stated absolute rule. – Uralite

@Uralite null isn't an object. See this. – Petta

@MattFenwick I should have just said "nulls are falsy" and excluded the word "objects". Otherwise, I believe my comment still makes sense in context. Just because you have an x such that typeof x yields "object" is not a valid way to check that x is truthy. – Uralite

Answer 2

if(x) will run if x is truthy.

x is truthy if it's not falsey.

x is falsey if x is null, undefined, 0, "", false

So since new Boolean(false) is an Object and an Object is truthy, the block runs

Sampan

Answer 3

In the if(x) case, it's actually evaluating the default Boolean of the object named and not its value of false.

So be careful using Boolean objects instead of Boolean values. =)

Dicarlo

Answer 4

The following code uses a Boolean object. The Boolean object is false, yet console.log("Found") still executes because an object is always considered true inside a conditional statement. It doesn’t matter that the object represents false; it’s an object, so it evaluates to true.

var found = new Boolean(false);
if (found) 
{    console.log("Found");
       // this executes
}

Skit