Some values may be one of multiple possible types. TypeScript refers to these “either or” types as type unions. This refers to the set theory concept of being a “union” of multiple possible types.


A union type is written as a series of other types with a | vertical pipe between them.

let myVar: number | string = '42';

Union Example

For example, the following logEither() function takes in a value that can be either a number or string:

function logEither(value: number | string) {
logEither('Durandal'); // Ok
logEither(7777777); // Also Ok
// Error: Type 'boolean' is not assignable to type 'number | string'.

Optional values, including optional parameters and members, are implicitly type | undefined.

Union type Members

When a value is a union type, TypeScript will only allow access to members that exist on all possible types.

In this example, we use a pair of ternary operators to create a value. This set the type of value to boolean | string | string[] and the only shared member is the toString() function. No others, such as length, are allowed since it can’t be applied to a boolean-type value:

// Type: boolean | string | string[]
const value =
Math.random() > 0.5
? false
: Math.random() > 0.5
? 'Leela'
: ['Durandal', 'Thoth'];
value.toString(); // Ok: exists on all three
* Error: Property 'length' does not exist
* on type 'false | "Leela" | string[]'.
* Property 'length' does not exist on type 'false'.

See Narrowing for how to narrow down union types into their more specific types.

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