Which effect has the extends constraint with a equality sign in a generic type definition?

Question

I've seen this extends syntax:

export type MyType<Data extends Base = Base> = { data: Data }

What does the usage of Base = Base do?

The TypeScript documentation about generics doesn't explain it.

Answer 1

Default generic parameter - docs

It allows you to call MyType without generic parameter.

type Base = { tag: 'Base' }

export type MyType<Data extends Base = Base> = { data: Data }

// type WithArgument = {
//     data: {
//         tag: 'Base';
//         name: 'John';
//     };
// }
type WithArgument = MyType<{ tag: 'Base', name: 'John' }>

// { data: Base; }
type Result = MyType 

Pure js analogy:

const myType = (arg = { tag: 'Base' }) => arg
const result = myType() // { tag: 'Base' }

This Data extends Base = Base means:

1. Data should extends Base
2. If Data is not provided, use Base by the default

This is how it can be splited into two parts:

1. Data extends Base
2. Data = Base

You should not read Base = Base separately from extends word