What is the logic of intersection and union types?

Question

typescript

As I understand it, after reading about these types, they should be treated as if they were sets. That is | | can merge two sets, and & finds their intersection. This example confirms this logic. Using an enum, the string type is discarded

type A = number | string;
type B = A & number; // number

However, I have a question when considering the second case. If we take objects instead of primitive types. Here & no longer finds an intersection ({ a: number }), but combines two objects. Which seems like a mistake to me. I know I'm wrong somewhere, but I can't figure out where.

type C = { a: number } | { b: number };
type D = C & { a: number };

const test2: D = {
  a: 1,
  b: 2
}

Answer 1

You misunderstood. Just read | as "or" and & as "and".
number | string means that the variable contains either a number or a string.
number &string means that the variable will be both a number and a string at the same time. And it does not matter how it is implemented, and whether it is feasible in principle (no).