function hello(callback, lastName) {
  console.log(callback);
  console.log('hello' + callback(lastName));
}

"takutaku"を返す関数
function getName() {
  return 'takutaku';
}

'yoo'を返す関数(アロー関数)
const getFirstName = function () {
  return 'yoo';
};

３行目の出力結果としてhelloyooが出力される
hello(getFirstName);

上と一緒hellohoge
hello(function () {
  return 'hoge';
});

hellofoohaaが出力
hello((name) => 'foo' + name, 'haa');



const doSomething = (a, b, callback) => {
  const result = callback(a, b);
  console.log(result);
};

const multiply = (a, b) => {
  return a * b;
};

const plus = (a, b) => {
  return a + b;
};

doSomething(2, 2, multiply); //4
doSomething(2, 3, plus);//5