typescript generic type with equal operator means?

Question

I am learning typescript generic, and come across the following generic type with the equal operator for extends type

export interface DataType {
  [key: string]: FieldValue;
}

export interface FormProps<Data extends DataType = DataType> { }

What does DataType = DataType mean in here?

Answer 1

If you don't provide a type Data (which must extend DataType), it will default to DataType.

From previous release notes

Consider a function that creates a new HTMLElement, calling it with no arguments generates a Div; you can optionally pass a list of children as well. Previously you would have to define it as:

declare function create(): Container<HTMLDivElement, HTMLDivElement[]>;
declare function create<T extends HTMLElement>(element: T): Container<T, T[]>;
declare function create<T extends HTMLElement, U extends HTMLElement>(element: T, children: U[]): Container<T, U[]>;

With generic parameter defaults we can reduce it to:

declare function create<T extends HTMLElement = HTMLDivElement, U = T[]>(element?: T, children?: U): Container<T, U>;

A generic parameter default follows the following rules:

• A type parameter is deemed optional if it has a default.
• Required type parameters must not follow optional type parameters.
• Default types for a type parameter must satisfy the constraint for the type parameter, if it exists.
• When specifying type arguments, you are only required to specify type arguments for the required type parameters. Unspecified type parameters will resolve to their default types.
• If a default type is specified and inference cannot chose a candidate, the default type is inferred.
• A class or interface declaration that merges with an existing class or interface declaration may introduce a default for an existing type parameter.
• A class or interface declaration that merges with an existing class or interface declaration may introduce a new type parameter as long as it specifies a default.