Intersection types in TypeScript

This entry is part 2 of 4 in the TypeScript series

Intersection types

Purpose of this post is to get a basic understanding of intersection types and realize pros and cons of intersection types.

Last time we talked about union types:

let x: string | number;

Variable x can be either string or a number. Consequently, you can use type guarding technique to exclude one of the types.

TypeScript also lets you define intersection types:

type PQ = P & Q;

let x: PQ;

Therefore, variable x has all properties from both P and Q.

Don’t let the intersection term lead you in wrong direction and confuse the logic with sets in mathematics.

In TypeScript if one type is intersection of two other types consequently that type will have all properties from two intersected types:

 

Be careful when mixing types that share some of the property names that don’t have same type:

You will notice that type X and type Y both have property c. However, type of the property is not the same and once we try to assign value to it we get an error from the compiler.

 

We can combine non primitive property types with the same property name:

As you can see we got interfaces A, B, C and all have same property name – x. However, type of the property x is different for those interfaces ( A, B, C ).

As a result we can combine those types to form a new type and use it:

let abc: ABC = {
 x: {
 d: true',
 e: 'codingblast',
 f: 3
 }
};

 

Do notice that we have 3 different properties on property x of object abc:

 

Type Relationships

It is important to notice that when you intersect types order does not matter:

type XY = X & Y;
type YX = Y & X;

Both,XY and YX have the same properties and are almost equal.

Also, (X & Y) & Z is equivalent to X & (Y & Z).



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Ibrahim Šuta

Software Engineer, Mentor and Consultant interested and specialising in ASP.NET Core, C#, JavaScript, Angular, React.js. Experienced in MeteorJS, Node.js, ASP.NET, Windows Forms and Web Forms as well.