What's the reason behind the usage of `declare` being allowed in class properties, but not in methods

class A {
    // specified explicitly for properties only
    declare foo(): string;
    // allowed for a property but not a method declaration
    declare bar: string;
}

What is the reasoning behind this specific restriction? Can someone provide an explanation?

I am curious about why this distinction exists, what purpose does it serve?

Playground link


PS: This should not be confused with:

class A {
    declare foo: () => string;
}

This is a property of a function type. It can be assigned unlike a method. One could argue for adding readonly, but that might introduce complexity.

Answer №1

There is no specific requirement for the limitation; it's simply that the functionality you are seeking has not been developed yet. In other words, it's a missing feature rather than a deliberate design choice.

Both microsoft/TypeScript#38008 and microsoft/TypeScript#48290 are active feature requests to allow the use of the `declare` modifier on class methods. If you wish to see this feature added, consider visiting these issues, giving them a thumbs up, and possibly sharing your own use case in the comments (especially if it offers additional insight). However, keep in mind that the outcome may not be affected much, as there are numerous open feature requests with higher user demand. Nevertheless, if more users express interest, the chances of implementation increase.

Until this feature is implemented, unfortunately, you will need to find alternative solutions through various techniques, which may or may not be adequate for some scenarios.

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