这是您想要的:

function alpha(a, b, fn) {
    console.log(a, b, a + b);
    fn();
}
alpha(5, 10, () => {
    console.log("hello");
});

// or defined by default
function alpha2(a, b, fn = () => {
    console.log("hello");
}) {
    console.log(a, b, a + b);
    fn();
}
alpha2(5, 10);