const response = {
  settings: {
    height: 400,
    animationDuration: 0,
    headerText: '',
    showSplashScreen: false
  }
};

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

console.log('headerText', headerText); // ''
console.log('animationDuration', animationDuration); // 0
console.log('showSplashScreen', showSplashScreen); // false