asynccompare.js

// With async

function foo(callback) {
    async.series([
        function (done) {
            one("x", done);
        },
        function (done) {
            if (FOO) {
                two("y", done);
            } else {
                done();
            }
        }
    ], callback);
}

// With .then()

function foo() {
    return one("x")
        .then(() => {
            if (FOO) {
                return two("y");
            }
        });
}

// With async/await

async function foo() {
    await one("x");

    if (FOO) {
        return await two("y");
    }
}