Union types

TypeScript allows you to declare union types:

let path: string[]|string;
path = "/temp/log.xml";
path = ["/temp/log.xml", "/temp/errors.xml"];
path = 1; // Error

In the preceding example, we have declared a variable named path that can contain a single path (string) or a collection of paths (array of strings). In the example, we have also set the value of the variable. We assigned a string and an array of strings without errors; however, when we attempted to assign a numeric value, we got a compilation error, because the union type didn't declare a number as one of the valid types of the variable.

Union types are used to declare a variable that can store a value of two or more types. Only the properties available in all the types present in the intersection type are considered valid:

We can appreciate this behavior in the following example:

interface Supplier {
 
    orderItems(): void; 
    getAddress(): void; 
} 
 
interface Customer { 
    sellItems(): void; 
    getAddress(): void; 
} 
 
declare let person: Supplier | Customer; 
person.getAddress(); // OK 
person.orderItems(); // Error 
person.sellItems();  // Error