How to combine object properties in typescript?

I'd like to know the best way to do this, say I have two objects

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

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

If objectC 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'd like to find a way without the need of defining interfaces for objectA and objectB. I don't want to write 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?

Seems like this should do the trick:

var objectA = {
    propertyA: 1,
    propertyB: 2,
    .
    . // more properties here
    .
    propertyM: 13
};

var objectB = {
    propertyN: 14,
    propertyO: 15,
    .
    . // more properties here
    .
    propertyZ: 26
};

var objectC = {...objectA, ...objectB}; // this is the answer

var a = objectC.propertyA;

var n = objectC.propertyN;

Based on this article: https://blog.mariusschulz.com/2016/12/23/typescript-2-1-object-rest-and-spread

In addition, the order of the variables in the decomposition matters.
Consider the following:

var objectA = {
    propertyA: 1,
    propertyB: 2, // same property exists in objectB
    propertyC: 3
};

var objectB = {
    propertyX: 'a',
    propertyB: 'b', // same property exists in objectA
    propertyZ: 'c'
};

// objectB will override existing properties, with the same name,
// from the decomposition of objectA
var objectC = {...objectA, ...objectB}; 

// result: 'b'
console.log(objectC.propertyB); 

// objectA will override existing properties, with the same name,
// from the decomposition of objectB
var objectD = {...objectB, ...objectA}; 

// result: '2'
console.log(objectD.propertyB);

Stack Overflow Q#37042602, score: 298