Когда знаешь как setTimeout работает в Javascript

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let header = document.querySelector('.header');
let elem = document.querySelector('.elem');


setTimeout(() => header.style.opacity = '0', 1000);
setTimeout(() => header.style.opacity = '0.15', 1100);
setTimeout(() => header.style.opacity = '0.2', 1200);
setTimeout(() => header.style.opacity = '0.25', 1300);
setTimeout(() => header.style.opacity = '0.3', 1400);
setTimeout(() => header.style.opacity = '0.35', 1500);
setTimeout(() => header.style.opacity = '0.4', 1600);
setTimeout(() => header.style.opacity = '0.45', 1700);
setTimeout(() => header.style.opacity = '0.5', 1800);
setTimeout(() => header.style.opacity = '0.55', 1900);
setTimeout(() => header.style.opacity = '0.6', 2000);
setTimeout(() => header.style.opacity = '0.65', 2100);
setTimeout(() => header.style.opacity = '0.7', 2200);
setTimeout(() => header.style.opacity = '0.8', 2300);
setTimeout(() => header.style.opacity = '0.9', 2400);
setTimeout(() => header.style.opacity = '1', 2500);