Can I determine if a value matches a type alias?

Question

In TypeScript, can I determine if a value is/matches a type alias?

Say I have this type:

export type Name = "Jane" | "John";

Then somewhere else I want to check if a certain piece of user input is of the Name type. Something like if (input instanceOf Name) won't work.

Is this even possible?

Answer 1

You can't check if a value matches a type alias. Types are erased at runtime, so any runtime code can't ever depend on them.

If you control the type alias I would recommend creating an array to hold the values, let TS infer the type for it, and derive the union from it. You can then check if a value is in the array:

const Name = ["Jane", "John"] as const
export type Name = typeof Name[number];

function isName(a: unknown): a is Name {
    return Name.indexOf(a as Name) != -1;
}

Playground Link

Answer 2

The only way that I am aware so far is generating the type alias from a const array, then checking your name against this array.

const names = ["Jane", "John"] as const;
type Name = typeof names[number];

function isValidName(name: string): name is Name {
    return names.includes(<Name> name);
}

Usage:

const test: string = "John";
if (isValidName(test)) {
    console.log(`${test} is a name`);
}
else {
    console.log(`${test} is not a name`);
}

I still can't understand why typescript can't compile from something like

type Example = 'first' | 'second';
const exampleValues = [typeof Example] as const;

to

const exampleValues = ['first', 'second'];

the answer AFAIK is that "Types are erased at runtime", but compiling a type alias to a literal array does not require type information during runtime.

1. it imposes no overhead
2. preserves runtime behavior of javascript
3. and does not provide additional runtime functionality

this would allow for developers to validate a string at runtime by looking for a value inside the array.

If there is a way to accomplish this please let me know. As I showed you can do the opposite, from an array to a type alias, but with type alias you can filter and combine types, and I am interested in converting from a type alias to an array at compile time.