First, I had to refresh my memory on the new object definition shorthand: In short, you can use a variable or expression to define a key name by using brackets, like: let key = "foo"; { [key]: "bar"}, and secondly you don't have to write { "baz" : function(p) { ... } }, you can instead write { baz(p) {...} }. OK, got it.
So, if I'm looking at the above example correctly, they're implementing what is essentially an Interface-based definition of a new "resource" object. (If it walks like a duck, and quacks...)
To make a "resource", you'll tack on a new magical method to your POJO, identified not with a standard name (like Object.constructor() or Object.__proto__), but with a name that is a result of whatever "Symbol.dispose" evaluates to. Thus the above definition of { [Symbol.dispose]() {...} }, which apparently the "using" keyword will call when the object goes out of scope.
Do I understand that all correctly?
I'd think the proper JavaScript way to do this would be to either make a new object specific modifier keyword like the way getters and setters work, or to create a new global object named "Resource" which has the needed method prototypes that can be overwritten.
Using Symbol is just weird. Disposing a resource has nothing to do with Symbol's core purpose of creating unique identifiers. Plus it looks fugly and is definitely confusing.
Is there another example of an arbitrary method name being called by a keyword? It's not a function parameter like async/await uses to return a Promise, it's just a random method tacked on to an Object using a Symbol to define the name of it. Weird!
JS has used "well-known symbols"[1] to allow extending / overriding the functionality of objects for about 10 years. For example, an object is an iterable if it has a `[Symbol.iterator]` property. Symbols are valid object keys; they are not just string aliases.
Symbols are a very safe way to introduce new "protocols" in either the language standard or for application code. This is because Symbol can never conflict with existing class definitions. If we use a string name for the method, then existing code semantics change.
Here are the well-known symbols that my NodeJS 22 offers when I `Symbol.<tab>`:
> identified not with a standard name (like Object.constructor() or Object.__proto__)
__proto__ was a terrible mistake. Google “prototype pollution”; there are too many examples to link. In a duck-typed language where the main mechanism for data deserialization is JSON.parse(), you can’t trust the value of any plain string key.
> To make a "resource", you'll tack on a new magical method to your POJO, identified not with a standard name [...] nothing to do with Symbol's core purpose of creating unique identifiers.
The core purpose and original reason why Symbol was introduced in JS is the ability to create non-conflicting but well known / standard names, because the language had originally reserved no namespace for such and thus there was no way to know any name would be available (and not already monkey patched onto existing types, including native types).
> Is there another example of an arbitrary method name being called by a keyword? It's not a function parameter like async/await uses to return a Promise, it's just a random method tacked on to an Object using a Symbol to define the name of it. Weird!
`Symbol.iterator` called by `for...of` is literally the original use case for symbols.
> I'd think the proper JavaScript way to do this would be to either make a new object specific modifier keyword like the way getters and setters work, or to create a new global object named "Resource" which has the needed method prototypes that can be overwritten.
Right, so instead of just grabbing a property by its well know unique name aka the entire point of symbols you need to first resolve the property descriptor, check a new completely unnecessary flag, and only then can you call the method. And of course that breaks if anyone either forgets the attribute or fails to check it when calling the parent method.
And you’re now wasting an entire keyword on a property with a fixed name, and code bases which already use that name with different semantics are unable to add `using` compatibility.
I hadn't considered how blessed I was to have __enter__ / __exit__ in Python for context managers and the more general Protocol concept that can be used for anything because lordy that definition looks ugly as sin. Even Perl looks on in horror of the sins JS has committed.
> Even Perl looks on in horror of the sins JS has committed.
LOL. What a great line.
It's getting pretty ridiculous, I agree. I don't understand the need for so many shorthands for one. All they do is make code illegible for the sake of saving a few keystrokes. Using the Symbol object that way is just ugly.
So, if I'm looking at the above example correctly, they're implementing what is essentially an Interface-based definition of a new "resource" object. (If it walks like a duck, and quacks...)
To make a "resource", you'll tack on a new magical method to your POJO, identified not with a standard name (like Object.constructor() or Object.__proto__), but with a name that is a result of whatever "Symbol.dispose" evaluates to. Thus the above definition of { [Symbol.dispose]() {...} }, which apparently the "using" keyword will call when the object goes out of scope.
Do I understand that all correctly?
I'd think the proper JavaScript way to do this would be to either make a new object specific modifier keyword like the way getters and setters work, or to create a new global object named "Resource" which has the needed method prototypes that can be overwritten.
Using Symbol is just weird. Disposing a resource has nothing to do with Symbol's core purpose of creating unique identifiers. Plus it looks fugly and is definitely confusing.
Is there another example of an arbitrary method name being called by a keyword? It's not a function parameter like async/await uses to return a Promise, it's just a random method tacked on to an Object using a Symbol to define the name of it. Weird!
Maybe I'm missing something.