Union Types

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Union Types

TypeScript’s type system allows you to build new types out of existing ones using a large variety of operators. Now that we know how to write a few types, it’s time to start combining them in interesting ways.

The first way to combine types you might see is a union type. A union type is a type formed from two or more other types, representing values that may be any one of those types. We refer to each of these types as the union’s members.

function printId(id: number | string) {
  console.log("Your ID is: " + id);
}
// OK
printId(101);
// OK
printId("202");
// Error
printId({ myID: 22342 });
Argument of type '{ myID: number; }' is not assignable to parameter of type 'string | number'.

Working with Union Types

It’s easy to provide a value matching a union type - simply provide a type matching any of the union’s members. If you have a value of a union type, how do you work with it?

TypeScript will only allow an operation if it is valid for every member of the union. For example, if you have the union string | number, you can’t use methods that are only available on string:

function printId(id: number | string) {
  console.log(id.toUpperCase());
Property 'toUpperCase' does not exist on type 'string | number'.
  Property 'toUpperCase' does not exist on type 'number'.
}

The solution is to narrow the union with code, the same as you would in JavaScript without type annotations. Narrowing occurs when TypeScript can deduce a more specific type for a value based on the structure of the code.

function printId(id: number | string) {
  if (typeof id === "string") {
    // In this branch, id is of type 'string'
    console.log(id.toUpperCase());
  } else {
    // Here, id is of type 'number'
    console.log(id);
  }
}

Another example is to use a function like Array.isArray:

function welcomePeople(x: string[] | string) {
  if (Array.isArray(x)) {
    // Here: 'x' is 'string[]'
    console.log("Hello, " + x.join(" and "));
  } else {
    // Here: 'x' is 'string'
    console.log("Welcome lone traveler " + x);
  }
}