union type check

union_type_check.ts

type typeA = { a: string };
type typeB = { b: string };
type typeAorB = typeA | typeB;

// type check for union type
function isA(foo: typeA | typeB): foo is typeA {
  return (<typeA>foo).a !== undefined;
}

// usage 
const foo: typeAorB[] = [{ a: '123' }, { b: '234' }];
foo.forEach(f => {
    if (isA(f)) {
        console.log(f.a);
    } else {
        console.log(f.b);
    }
})