What is a promise entry?

Question

JavaScript

What is this promise entry form - how does it work?

layerSearchActions: { [layer: number]: (items: LayerItemGeneric[], search: string) => Promise<{ [key: string]: LayerWellTool }>;} = {
    335: async (
      items: LayerItemGeneric[],
      search: string
    ): Promise<{ [key: string]: LayerWellTool }> => {

 }
  };

Can explain step by step, for the first time I meet such a record

Answer 1

A promise that resolves to an object, with type keys stringand valuesLayerWellTool

const foo: Promise< тут указывается тип данных которые возвращаются из промиса >

Answer 2

{ [layer: number]: (items: LayerItemGeneric[], search: string) => Promise<{ [key: string]: LayerWellTool }>;}

it is written here that the object of such an interface contains properties of the number type, the value of which is a function with parameters that returns a promise,
the fields are respectively items, this is an array of elements of the LayerItemGeneric type and a certain search string,
the return value of the promise is used as a generic, it is shown that the returned the result will be some object whose properties will be some strings whose value will (each) be something of type LayerWellTool