let a: string | undefined;

function echo<T>(a: T): T {
    return a;
}

function add(a: string): string {
    return `${a}1`;
}

let b = echo(a);
const c = echo(a);

if (!b || !c) {
    return;
}

const why = function () {
    console.log(add(b)); // Argument of type 'string | undefined' is not assignable to parameter of type 'string'.Type 'undefined' is not assignable to type 'string'
    console.log(add(c));
};