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typescripttype-alias

Can I determine if a value matches a type alias?

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?

Solution

  • 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