Branching by type in a generic way

[LeaVerou]

In the initial Treecle port, I removed all the functionality that supports type => something object literals and instead it only supports functions for these things. I instead added code to Vastly to transform object literals into functions.

But it just dawned on me: this would be very useful in Treecle too, it's just that we can't depend on node.type returning a useful type. But we could have a config.getType() method, which by default returns the object [[ Class ]], but can be overridden (e.g. Vastly would do config.getType = n => n.type).

Any objections @adamjanicki2 ?

[adamjanicki2]

No objections here, I think that sounds good. What does [[ Class ]] mean?

[LeaVerou]

What does [[ Class ]] mean?

Warning: infodumping ahead 😅

When you do Object.prototype.toString.call(obj), you get a string like "[object Foo]". The Foo in there is the object’s internal type, which used to be called [[ Class ]]. You may still see "[[ Class ]] being used since AFAIK there is no better term to describe the concept).

Reading [[ Class ]] is generally more robust than typeof or instanceof, since:

• typeof does not give you useful info when you’re dealing with different types of objects (eveyrthing is just "object") and is thrown by primitive wrapper objects, i.e. typeof new String("foo") is "object", not "string"
• Because globals are tied to their originating globalThis, the same constructor objects from different contexts will not be equal. E.g. if you grab a reference to an array from an <iframe> and try to do foo instanceof Array in the parent frame, it will be false, because it's an instance of a different Array class. That's why we got Array.isArray() eventually, because simply checking if something is an array was incredibly painful if you want to do it in a robust way (and while you can cut corners on robustness when working on an app you control, since you know what code you're dealing with, you can't when developing a library that needs to work in environments you don't control.

In the past only native objects had the privilege of having a [[ Class ]] that was not "Object", but now authors can use Symbol.toStringTag to override the default.

[[ ]] denotes an internal slot, i.e. a property or method that is implemented by the JS runtime but cannot be accessed or modified directly. See https://medium.com/jspoint/what-are-internal-slots-and-internal-methods-in-javascript-f2f0f6b38de

[adamjanicki2]

Following up on this, how do you get the [[ Class ]] for an object?

[LeaVerou]

Like this: https://github.com/mavoweb/treecle/blob/main/src/util.js#L19-L33

[adamjanicki2]

Accidentally closed this