Explain the distinction between
class Document<T extends {}>{}class Document<T>{}
Can you provide examples to illustrate the difference?
Explain the distinction between
class Document<T extends {}>{}class Document<T>{}
Can you provide examples to illustrate the difference?
A generic type parameter with no specific constraint is considered unconstrained, or has an implicit constraint to the unknown type to which all types are compatible. Therefore, in the examples like class Document<T>{} or
class Document<T extends unknown>{}T:
class Document<T>{ /**implementation involving T**/ }
new Document<{ a: string }>(); // valid
new Document<string>(); // valid
new Document<null>(); // valid
new Document<undefined>(); // valid
new Document<string | undefined>(); // valid
On the contrary, T extends {} limits T to the so-called "empty object type" {}, which includes any object type or primitive type that behaves like an object when indexed into it. If JavaScript automatically wraps a primitive type with a wrapper object, then that primitive type is assignable to {}.
The only primitives in JavaScript that do not get wrapped are null and undefined, making the TypeScript values not assignable to {} from the null and undefined types.
Therefore, T extends {} allows T to be anything except for those with null or undefined in its domain:
class Document<T extends {}>{ /**implementation involving T**/ }
new Document<{ a: string }>(); // valid
new Document<string>(); // valid
new Document<null>(); // error
new Document<undefined>(); // error
new Document<string | undefined>(); // error
It is a loose constraint compared to T extends unknown, but it adds some level of restriction as demonstrated.
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