Problem

I have this object:

Both are the same type, however, they can be undefined, so what is the best and cleanest approach to merging them?

I thought something like these:

But it doesn't work, I'm getting this error:

Solution

Since the properties are optional, cart.orderPromo is either an array of ProductPromotion, or undefined (as indicated by the error message). In the latter case, the use of the >spread operator ...cart.orderPromo isn't possible.

You should therefore convert the properties to empty arrays if they are undefined, typically 

with the >null coalescing operator ??:

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