TypeScript Union Types

Union types are used when a value can be more than a single type.

Such as when a property would be string or number.

Union | (OR)

Using the | we are saying the parameter is a string or number:

Example

function printStatusCode(code: string | number) {

  console.log(`My status code is ${code}.`)

}

printStatusCode(404);

printStatusCode('404');

Union Type Errors

Note: you need to know what your type is when union types are being used to avoid type errors:

Example

function printStatusCode(code: string | number) {

  console.log(`My status code is ${code.toUpperCase()}.`) // error: Property 'toUpperCase' does not exist on type 'string | number'. Property 'toUpperCase' does not exist on type 'number'

}

In the example we are having an issue invoking toUpperCase() as its a string method and number doesn't have access to it.