Typescript Check If A String Exists as An Interface Key

user_3464127 0
Viewed 5884

Can I check if a string exists as an interface key

interface list {
    one: string
    two: string
}

const myNumber = "one"

How do I check if myNumber value is an interface key

typescript interface
2 Answers

To do this, you'll need to have something that lets you get the key of the interface at runtime. An interface does not exist at runtime - it's purely a TypeScript construct, so it doesn't exist in emitted code.

Make an array that contains the keys, declare it as const so it doesn't get automatically type-widened, and then you'll be able to turn it into the List type. Then you'll have both a type and a runtime array that you can use an .includes check on:

const listKeys = ['one', 'two'] as const;
type List = Record<typeof listKeys[number], string>;

// ...

const obj = {
    one: 'one',
    two: 'two',
    three: 'three'
};
// Transformation to string[] needed because of an odd design decision:
// https://github.com/Microsoft/TypeScript/issues/26255
const newObj = Object.fromEntries(
    Object.entries(obj).filter(
        ([key]) => (listKeys as unknown as string[]).includes(key)
    )
);

Playground link

user_95152070

The type of Typescript is not a value.

Therefore, operation of Javascript is impossible.

However, in the example, the type can be set so that myNumber is the type corresponding to the key.

interface list {
    one: string
    two: string
}

const myNumber: keyof list = "one"; // myNumber allow only "one" or "two";
user_108664090
Related