CODEWITHSUNDEEP
javascript
InsOp
InsOp

Reputation: 2699

why does accessing a property of a boolean value not throw an error - javascript

Why does accessing false.bar not throw an error while undefined.bar does?

var foo1 = false;
var foo2;
console.log(foo1.bar); // undefined
console.log(foo2.bar); // Uncaught TypeError: Cannot read property 'bar' of undefined

Upvotes: 0

Views: 344

Answers (3)

Touffy
Touffy

Reputation: 6561

Look around for "boxing", you'll find that many answers have already been written.

To sum it up, Boolean, Number and String instances will magically "box" primitive values in a context where you're treating them as objects. Booleans don't have any useful methods or properties except the default Object-inherited ones, so you'll never see boolean boxing. But the language allows it.

However, there are no Null or Undefined constructors to box those values, so trying to access properties of null and undefined is an error.

Upvotes: 1

msaw328
msaw328

Reputation: 1569

It's because false itself is an object, while undefined isn't.

Variable is undefined if it wasn't assigned a value yet, like foo2 in your code. You're trying to access a property of something that doesn't exist, so JS gets scared and throws error.

When you're trying to access foo1, the object foo1 exists, since you've assigned false to it. JS doesn't mind that this object doesn't have a "bar" property and will just return undefined, as in "this property isn't defined yet".

Upvotes: 0

80prozent
80prozent

Reputation: 167

In your example, foo1 actually exists, so we can check for a property on it (it has no property defined called "bar", so we get a undefined for that).

The other case, foo2, doesnt exists (it never has been init and is still undefined), so trying to check for a property will result in a error.

Upvotes: 0

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