Date.prototype.getMonth()

const moonLanding = new Date("July 20, 69 00:20:18");
console.log(moonLanding.getMonth()); // (January gives 0)
// 予想される結果: 6

getMonth()

const valentines = new Date("1995-02-14");
const month = valentines.getMonth();
const monthNames = ["January", "February", "March" /* , … */];
console.log(monthNames[month]); // "February"

const options = { month: "long" };
console.log(new Intl.DateTimeFormat("en-US", options).format(valentines));
// "February"
console.log(new Intl.DateTimeFormat("de-DE", options).format(valentines));
// "Februar"

const xmas95 = new Date("1995-12-25T23:15:30");
const month = xmas95.getMonth();
console.log(month); // 11