What is Union Type

In TypeScript, a union type is a type that allows a value to be one of several specified types. It’s useful when you want to allow a variable to have multiple potential types. You define a union type using the vertical bar (|) to separate each type.

Example of Union Type

let data: string | number;
data = "Hello World"; // OK
data = 100;      // OK
data = true;    // Error: Type 'boolean' is not assignable to type 'string | number'.

In this example, the variable data can be either a string or a number, but not any other type like boolean.

Usage in Functions

function format(input: string | number): string {
  if (typeof input === "string") {
    return input.toUpperCase();
  } else {
    return input.toFixed(2);

console.log(format("Hello World")); // Output: "HELLO WORLD"
console.log(format(120));      // Output: "120.00"

Here, the format function accepts either a string or a number as its input and handles each type appropriately.

Union Types with Objects

type Cat = { name: string; purrs: boolean };
type Dog = { name: string; barks: boolean };

type Pet = Cat | Dog;
function makeSound(pet: Pet): string {
  if ("purrs" in pet) {
    return pet.name + " purrs.";
  } else {
    return pet.name + " barks.";

const cat: Cat = { name: "Pussy", purrs: true };
const dog: Dog = { name: "Tiger", barks: true };

console.log(makeSound(cat)); // Output: "Pussy purrs."
console.log(makeSound(dog)); // Output: "Tiger barks."