有没有一种类型级别的方法可以从类中提取具有默认值的属性？

Question

I want to extract the key names of properties in a class that have default values. Is this information available to the type system?

class Foo {
  hasDefault: boolean = true
  alsoHasDefault: number = 42
  noDefault: boolean
}

// How can this be implemented?
type DefaultPropertyNames<T> = ... 

// Example output:
type FooDefaults = DefaultPropertyNames<Foo> // -> 'hasDefault' | 'alsoHasDefault'

Answer 1

I think what you mean by "no default value" is that the value can be undefined. Currently, your code throws a type error because noDefault does not contain undefined in its type definition.

So let me reframe the question:

How do I find all of the keys in a class that are potentially undefined?

Something like this works:

type DefaultPropertyNames<T> = Exclude<{
    [K in keyof T]: {key: K, value: T[K] extends T[K] & {} ? true : false}
}[keyof T], {value: false}>["key"]

playground

To explain a little bit:

• T[K] & {} removes undefined from a type.

• T[K] extends T[K] & {} tells you if undefined is in the union type

• {[K in keyof T]: ... }[keyof T] creates a union type from each property key

• Exclude lets you remove items from the union type.