新的 Null 判断运算符??

新的 Null 判断运算符?? 它的行为类似||


？？ 只有运算符左侧的值为null或undefined时，才会返回右侧的值

||  es5除了 null或undefined ，空字符串或false或0，都会返回右侧的值　

const headerText = response.settings.headerText || 'Hello, world!';
const animationDuration = response.settings.animationDuration || 300;
const showSplashScreen = response.settings.showSplashScreen || true;

const headerText = response.settings.headerText ?? 'Hello, world!';
const animationDuration = response.settings.animationDuration ?? 300;
const showSplashScreen = response.settings.showSplashScreen ?? true;