Can you explain the meaning of <T = {}>?

Question

Can you explain the meaning of <T = {}>?

While browsing through the documentation, I came across this generic type:

type GConstructor<T = {}> = new (...args: any[]) => T;

https://www.typescriptlang.org/docs/handbook/mixins.html

Above this line, there is a brief mention that it is a Generic Constraint. However, details about this particular constraint are not provided in the official documentation on Generic Constraints: https://www.typescriptlang.org/docs/handbook/2/generics.html#generic-constraints

So my question is, how exactly does <T = {}> operate? And what specific constraints does it impose?

typescript

Answer 1

Answer №1

It is not a generic constraint; these utilize extends to ensure that the generic type parameter can be assigned to another type:

// the type T must be assignable to {a: string}
type Foo<T extends { a: string }> = T;

type A = Foo<{ a: "hello" }> // {a: "hello"}
type B = Foo<Date> // error!

Instead, it is a generic parameter default, which serves as a fallback type that the compiler uses if you do not specify or it cannot infer the type parameter:

// the type T will default to {a: string, b: number} if not specified:
type Bar<T = { a: string, b: number }> = T;
type C = Bar<Date> // Date
type D = Bar // {a: string, b: number}

These concepts are separate, but can be used together to constrain a type parameter and provide a default value (which must be assignable to the constraint itself):

// both
type Baz<T extends { a: string } = { a: string, b: number }> = T;
type E = Baz<{ a: "hello" }> // {a: "hello"}
type F = Baz<Date> // error!
type G = Baz // {a: string, b: number}

Link to Playground for code demonstration

Answer 2

It is not a generic constraint; these utilize extends to ensure that the generic type parameter can be assigned to another type:

// the type T must be assignable to {a: string}
type Foo<T extends { a: string }> = T;

type A = Foo<{ a: "hello" }> // {a: "hello"}
type B = Foo<Date> // error!

Instead, it is a generic parameter default, which serves as a fallback type that the compiler uses if you do not specify or it cannot infer the type parameter:

// the type T will default to {a: string, b: number} if not specified:
type Bar<T = { a: string, b: number }> = T;
type C = Bar<Date> // Date
type D = Bar // {a: string, b: number}

These concepts are separate, but can be used together to constrain a type parameter and provide a default value (which must be assignable to the constraint itself):

// both
type Baz<T extends { a: string } = { a: string, b: number }> = T;
type E = Baz<{ a: "hello" }> // {a: "hello"}
type F = Baz<Date> // error!
type G = Baz // {a: string, b: number}

Link to Playground for code demonstration

Can you explain the meaning of <T = {}>?

Answer №1

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