Is there a way to check if a string is included in a Union type in TypeScript?

Question

Consider the following code:

type Foo = "Foo" | "Bar" | "Baz"

function isInFoo(str: string) boolean {
    // return Foo.contains(str); ?
}

In typescript, is there an elegant way to check if str is in type Foo?

Answer 1

Type annotations are removed from compiled code and are not available at runtime. But to expand on Ivan's answer, here is an example of extracting typed data from an array:

const fooBar = ['foo', 'bar', 'baz'] as const;
type FooBar = typeof fooBar[number]; // "foo" | "bar" | "baz"

Then you can write a custom type guard that checks a string at runtime:

function isFooBar(string: unknown): string is FooBar {
    return typeof string === 'string' && string in fooBar;
}

And use it like this:

const maybeFooBar: unknown = 'baz';
if (isFooBar(maybeFooBar)) {
    console.log('Typescript knows this is a FooBar');
}