typescript interface dynamic param does not compile without any

If you define an indexer all properties must conform to the return type of the interface. You could do the following:

export interface Aggr {    [name: string]: AggrEntry |Aggr;    filter?: Aggr;}

This is obviously not ideal as this allows other fields except filter to be of type Aggr.

Another option is to use a type definition instead of an interface, and use a intersection type:

export type Aggr  = {    [name: string]: AggrEntry;} & {    filter?: Aggr;}let test : Aggr;let foo = test.foo // foo is AggrEntrytest.filter // works, is of type Aggr

While we can access object fields as expected, creating an object of this type is a bit trickier. Creating an object literal with the filter field will yield a similar error to your original. We can use Object.assign to create an instance of the type using object literals:

let test : Aggr = Object.assign({    foo: new AggrEntry()}, {    filter: {        bar: new AggrEntry()    }});

Or we can create a dedicated function to help with creation, that uses Object.assign:

function createAggr(dynamicPart: {    [name: string]: AggrEntry;}, staticPart?: {    filter?: Aggr;}) {    return Object.assign(dynamicPart, staticPart);}let test : Aggr = createAggr({    foo: new AggrEntry()}, {    filter: {        bar: new AggrEntry()    }});