How to use same technique as ts omit, but applying it to object of concrete types?

Question

I basically want to implement left | right join of two objects looking at their types. For example, object config of default type RequestInit.

config = {
   headers: {
    'Content-Type': 'application/json',
   },
}

And I have a customized customConfig of type, for example, CustomRequestInit that contains additional query params, which I later will join with url.

customConfig = {
   mode: 'cors',
   headers: {
    'Content-Type': 'application/json',
   },
   params: {
    'tk': '2edr3q2afa3',
   }
}

This object has properties from RequestInit class, they should overwrite same properties in config object. Other properties from customConfig mustn't be in the final config object. How do I left join this two objects by their types, ommitting additional keys that CustomRequestInit brings in?

Answer 1

It is a bit unclear what you are trying to achieve.

This type adds properties from B which are missing in A (only adds new properties).

type AddMissing<A, B> = A & Omit<B, keyof A>;