isT.ts

interface A { a: number }
interface B { b: number }

function ifIs<T>(l: Array<A | B>, isT: (o: any) => o is T, handle: (t: T) => void) {
    for (const o of l) {
        if (isT(o)) {
            handle(o);
        }
    }
}

ifIs([{a: 5}, {b: 6}], isB, (b: B) => console.log(b.b));

function isA(o: any): o is A {
    return 'a' in o;
}

function isB(o: any): o is B {
    return 'b' in o;
}