can typescript do union type assertion in this case?

Question

can typescript do union type assertion in this case? i want to use ab.a or ab.b or ab. hasOwnProperty ab. hasOwnProperty to do assertion of type A or type B ? how do i do?

export interface A extends Object {
    a: string;
}

export interface B extends Object {
    b: number;
}

export type AorB = A | B;

function test(ab: AorB) {
    // can ts auto predict this ?
    if (ab.hasOwnProperty('a')) {
        ab.a // type error
    }
}

Answer 1

Update your function as below:

function test(ab: AorB) {
    // can ts auto predict this ?
    if ('a' in ab) {
        console.log(ab.a);
    }
}