const from = '<https://www.example.com/courses/39/units/8570>';

/**
 * 不寫是當前目錄
 * => <https://www.example.com/courses/39/units/foo> */
console.log(new URL('foo', from).toString());

/**
 * / 為根目錄
 * => <https://www.example.com/foo> */
console.log(new URL('/foo', from).toString());

/**
 * ./ 與不寫相同
 * => <https://www.example.com/courses/39/units/foo> */
console.log(new URL('./foo', from).toString());

/**
 * ../ 為上一層目錄
 * => <https://www.example.com/courses/39/foo> */
console.log(new URL('../foo', from).toString());

/**
 * // 為同一個 protocol 的絕對位置
 * => <https://foo> */
console.log(new URL('//foo', from).toString());