How to Perform Runtime Type Checking for Interfaces in TypeScript?

Interface Type Check with TypeScript

Question

How can one perform runtime type checking for interfaces in TypeScript, considering that JavaScript lacks the concept of interfaces?

Answer

While you cannot use instanceof with interfaces in TypeScript, you can create custom type guards to achieve the desired behavior:

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);
}

For cases where multiple members need to be checked, consider introducing a discriminator property:

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);
}

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