The TypeScript type system offers a basic support for composite types:
- Union Types represent a value that can be one of the several types. They are constructed using a vertical bar (
|) like
the following type NumberOrString = number | string.
- Intersection Types combine multiple types into one, so that the object of such type will have all the members of all intersection type
elements. They are constructed using an ampersand (
&) like the following type SerializablePerson = Person &
Serializable. Intersection Types are often used to represent mixins.
Duplicating types when defining a union or interaction type makes the code less readable. Moreover duplicated types might be a simple mistake and
another type should be used instead.
Noncompliant Code Example
function padLeft(value: string, padding: string | number | string) { // Noncompliant; 'string' type is used twice in a union type declaration
// ...
}
function extend(p : Person) : Person & Person & Loggable { // Noncompliant; 'Person' is used twice
// ...
}
Compliant Solution
function padLeft(value: string, padding: string | number | boolean) {
// ...
}
function extend(p : Person) : Person & Loggable {
// ...
}
