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Narrow type of constructor.name #52909
Description
Suggestion
🔍 Search Terms
class
constructor.name
✅ Viability Checklist
My suggestion meets these guidelines:
- This wouldn't be a breaking change in existing TypeScript/JavaScript code
- This wouldn't change the runtime behavior of existing JavaScript code
- This could be implemented without emitting different JS based on the types of the expressions
- This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, new syntax sugar for JS, etc.)
- This feature would agree with the rest of TypeScript's Design Goals.
⭐ Suggestion
class Foo {}
type FooName = typeof Foo.constructor.name; // Should be literal `"Foo"`, but is instead `string`In the context of this issue, I am not sure whether this is a "bug" or a "suggestion", but I went with a suggestion for now.
📃 Motivating Example
I want to use TypeScript to ensure that all of my classes meet some criteria. Consider the following trivial example:
class Foo {}
class Bar {}
class Baz {}
/** We organize them into an array so that we can instantiate them all at the same time with some common parameters. */
const MY_CLASSES = [
Foo,
Bar,
Baz,
] as const;
/** Helper type to retrieve the class names of a tuple containing classes. */
type MyClassNames = InstanceType<typeof MY_CLASSES[number]>["constructor"]["name"]
// Oops! TypeScript messed up, and `MyClassName` is `string` instead of `"Foo" | "Bar" | "Baz"`.
const CLASSES_TO_METADATA = {
Foo: "something",
Bar: "something else",
} as const satisfies Record<MyClassNames, string>Oops, I forgot to put an entry for my Baz class inside of my CLASSES_TO_METADATA object. Even though I carefully took the time to specify a satisfies constraint, TypeScript doesn't care!
I do not think there are any workarounds for this "bug" right now other than to explicitly duplicate the name of the class inside of the class itself, like this:
class Foo {
readonly name = "Foo" as const;
}
class Bar {
readonly name = "Bar" as const;
}
class Baz {
readonly name = "Baz" as const;
}
/** We organize them into an array so that we can instantiate them all at the same time with some common parameters. */
const MY_CLASSES = [
Foo,
Bar,
Baz,
] as const;
/** Helper type to retrieve the class names of a tuple containing classes. */
type MyClassNames = InstanceType<typeof MY_CLASSES[number]>["name"]
const CLASSES_TO_METADATA = {
Foo: "something",
Bar: "something else",
} as const satisfies Record<MyClassNames, string>Very ugly! But in this second code snippet, the satisfies part works properly.
Activity
MartinJohns commentedon Feb 22, 2023
Duplicate of #37340.
Trying to narrow via
constructor.nameis a fairly bad choice. The class may be subclassed, in which case the name will be different, but it will still be a valid instance.Zamiell commentedon Feb 22, 2023
Ok. Forgetting
constructor.namefor a moment, is there a way to go fromFooto the name of the method, purely at compile-time, via some other method? I'm not interested in any run-time behavior, only compile-time.MartinJohns commentedon Feb 22, 2023
No. See #43325.