How to combine object properties in typescript

Question

If I have two items, what is the best approach to go about doing this?

var objectA = {
    propertyA: 1,
    propertyB: 2
    ...
    propertyM: 13
}

var objectB = {
    propertyN: 14,
    propertyO: 15
    ...
    propertyZ: 26
}

If object is created by

var objectC = Object.assign(objectA, objectB);

How can I declare/describe objectC, so the compiler/IDE knows that it has the properties of both objectA and objectB?

I want to find a way without the need to define interfaces for objectA and objectB. I don't want to write a declaration and definition/evaluation for the same property twice. This redundancy is significant if I have too many properties on an object.

(Is there an operator that can extract the interface/type of an existing object?)

Is it possible?