Typescript interface with named property and arbitrarily-named index properties of different type [duplicate]

Question

I would like to create an interface for an object that has a certain type for a specific named property and a different type for all other properties.

How would I write a definition for foo below?

let foo = {
   size: 3,
   a: 'foo',
   b: 'bar',
   c: 'baz'
}

This would be my intuitive approach:

interface Foo {
    size: number;
    [name: string]: string;
}

However, TypeScript tries to apply the general definition to the specific definition and triggers the following error:

error TS2411: Property 'size' of type 'number' is not assignable to string index type 'string'.

Answer 1

You can do something like this (I don't know if it will work in TS v1.5):

interface Size {
    size: number;
}

interface StringMap {
    [name: string]: string;
}

type Foo = Size & StringMap;

Answer 2

Without defining every property, that's expected, but you can do this:

interface Foo1 {
    size: number;
    [name: string]: string|number;
}

Better than using any.