Interface vs Types : intersection and union of properties and values

Issue

I have been reading book effective typescript and there is this paragraph there with code which is not clear to me :

1.

  interface Person {
      name: string;
    }
    interface Lifespan {
      birth: Date;
      death?: Date;
    }
    
    type PersonSpan = Person & Lifespan; // why not never type ?

The & operator computes the intersection of two types. What sorts of values belong to the PersonSpan type?

On first glance the Person and Lifespan interfaces have no properties in common, so you might expect it to be the empty set (i.e., the never type). But type operations apply to the sets of values (the domain of the type), not to the properties in the interface. And remember that values with additional properties still belong to a type. So a value that has the properties of both Person and Lifespan will belong to the intersection type:

const ps: PersonSpan = {
 name: 'Alan Turing',
 birth: new Date('1912/06/23'),
 death: new Date('1954/06/07'),
}; // OK

2. what if declare

   type newPersonSpan = Person | Lifespan; // what are the possible values we can assign now ?

could someone please explain this in more layman language

Solution

The short version is: to be a Person & Lifespan, an object must have all the properties defined in Person, and it must also have all the properties defined in Lifespan.

Long version: The book is describing one way you can conceptualize the way that & works: as the intersection of sets. First, let's start with what this means:

interface Person {
  name: string;
}

To be a Person, a variable must be an object, and that object must have a name property that is a string. Simple enough, but it's important to point out: it can also have any number of extra properties, and still be a Person. So all of the following objects are compatible with Person:

{ name: 'bob' }
{ name: 'alice', age: 30 }
{ name: 'eve', birth: new Date() }

You can think of types as being mathematical sets, in which case Person is the set of the 3 objects i just listed, plus every other possible object that would match the definition. All objects that have a string for a name, plus any number of extra properties of any type.

Similarly, Lifespan can also be thought of as a set. To be in the Lifespan set, an object must have a birth property which is a Date, and if it has a death property, that property must also be a Date. And as before, any number of extra properties are allowed

Examples of Lifespans include:

{ birth: new Date(), death: new Date() }
{ birth: new Date(), name: true }
{ name: 'eve', birth: new Date() }

The & operator finds the intersection of two sets. Some objects are in Person, some are in Lifespan, and some are in both; the intersection means the ones that are in both.

So from the examples i gave, { name: 'eve', birth: new Date() } is in the intersection. It is both a Person, and a Lifespan. The rest of the examples are only in one, not the other, for one reason or another. Eg, { name: 'bob' } is not a Lifespan because it has no birth, and { birth: new Date(), name: true } is not a Person because its name is the wrong type.

Answered By - Nicholas Tower