Intersection Types ('&')

Part of the Introduction To TypeScript section of Coddy's JavaScript journey — lesson 34 of 73.

While union types let you create variables that can be one of several types, intersection types work in the opposite direction. They combine multiple types into a single type that has all the properties of the combined types.

Intersection types use the ampersand (&) operator to merge types together. When you intersect two object types, the result contains every property from both types:

type HasName = { name: string };
type HasAge = { age: number };

type Person = HasName & HasAge;

// Person now has both name AND age properties
let user: Person = {
  name: "Alice",
  age: 30
};

This is particularly useful when you want to build complex types from smaller, reusable pieces. Instead of defining one large object type, you can create focused types for specific concerns and then combine them as needed.

Intersection types ensure that the resulting type satisfies all the requirements of the intersected types. The variable must have every property from every type in the intersection - there's no picking and choosing like with unions.

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

Challenge

Easy

Create two type aliases: HasName for an object with a name property of type string, and HasAge for an object with an age property of type number.

Create a third type alias called Person by intersecting HasName and HasAge using the & operator.

Create another type alias called HasEmail for an object with an email property of type string.

Create a fourth type alias called Employee by intersecting all three types: HasName, HasAge, and HasEmail.

Now create the following variables:

Create a variable named user of type Person with the name "Alice" and age 25
Create a variable named worker of type Employee with the name "Bob", age 30, and email "bob@company.com"

Create a function named displayPerson that accepts a parameter of type Person and returns a string in the format "Name: [name], Age: [age]".

Create another function named displayEmployee that accepts a parameter of type Employee and returns a string in the format "Name: [name], Age: [age], Email: [email]".

Call both functions with the appropriate variables and print the results on separate lines.

Cheat sheet

Intersection types combine multiple types into a single type that has all the properties of the combined types using the ampersand (&) operator:

type HasName = { name: string };
type HasAge = { age: number };

type Person = HasName & HasAge;

// Person now has both name AND age properties
let user: Person = {
  name: "Alice",
  age: 30
};

The resulting type must satisfy all requirements of the intersected types - every property from every type in the intersection is required.

Try it yourself

// TODO: Write your code here
// Create type aliases for HasName, HasAge, HasEmail
// Create Person and Employee types using intersection
// Create variables user and worker
// Create displayPerson and displayEmployee functions
// Call the functions and print the results

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

Intersection Types ('&')

Cheat sheet

All lessons in Introduction To TypeScript