Interface Type Check in TypeScript

Within TypeScript, you may encounter a scenario where determining an object's compliance with a predefined interface at runtime is crucial. While class type checks leveraging the instanceof keyword are straightforward, applying it to interfaces presents a challenge.

Traditional approaches, such as relying on the instanceof operator, prove ineffective since interfaces lack representation as distinct types in compiled JavaScript. Instead, custom type guards offer a solution:

interface A {
    member: string;
}

function instanceOfA(object: any): object is A {
    return 'member' in object;
}

var a: any = { member: "foobar" };

if (instanceOfA(a)) {
    alert(a.member);
}

This approach allows runtime verification of interface compliance without the need for the instanceof keyword.

In situations where multiple members must be checked, a discriminator property can be introduced. This approach requires managing your own discriminators and ensuring uniqueness to avoid conflicts:

interface A {
    discriminator: 'I-AM-A';
    member: string;
}

function instanceOfA(object: any): object is A {
    return object.discriminator === 'I-AM-A';
}

var a: any = { discriminator: 'I-AM-A', member: "foobar" };

if (instanceOfA(a)) {
    alert(a.member);
}

By employing custom type guards or discriminators, you can effectively perform interface type checks at runtime, enhancing the robustness of your TypeScript applications.