Intersection Type

A type combining all the members of several types.

Also known as: intersection, type intersection

An intersection type combines several types into one, so a value must satisfy all of them at once. In TypeScript you write it with &:

type Named = { name: string };
type Aged = { age: number };
type Person = Named & Aged;

const ada: Person = { name: "Ada", age: 36 };   // must have both name and age
// const bad: Person = { name: "Ada" };          // error: age is missing

A Person has every property of Named and every property of Aged. This is a convenient way to build a type from smaller pieces, especially when a function takes an argument that combines several capabilities.

The trade-off is readability and conflicts. When two members share a name with incompatible types, the result can become never, a type no value can have, and the error may appear far from the declaration. Long chains of intersections are also hard to read in editor hover text. An interface that extends another gives a similar result with a clearer error message when the types disagree.

The classic mistake is treating an intersection as a union, or assuming it’s a dictionary merge at runtime. The & only exists in the type system. The actual value is just one object, and nothing is combined when the code runs. Also, a type guard can narrow a value to an intersection, but it only checks what you tell it to check.