the-glorious-startpage/js/background-set.js

class DummyBodyBackground {

	constructor() {
		this._body = document.body;
		this._dummyBodyBackground = document.querySelector('#dummyBodyBackground');
		
		this._bodyStyle = this._body.style;
		this._dummyBodyBackgroundStyle = this._dummyBodyBackground.style;
		
		this._hqBackground = document.createElement('img');
		
		// Initialize
		this._startLazyLoad();
	}

	_styleBodyBackgrond = (elemStyle, urlStr) => {
		elemStyle.background = urlStr;
		elemStyle.backgroundSize = 'cover';
		elemStyle.backgroundRepeat =  'no-repeat';
		elemStyle.backgroundPosition = 'center';
		elemStyle.backgroundAttachment = 'fixed';
	}

	_lazyLoadBackground = fileName => {
				
		// Add a class to blur the dummy background
		this._dummyBodyBackground.classList.add('dummyBackgroundBlur');

		// Set a low quality background image for the dummy background
		this._styleBodyBackgrond(this._dummyBodyBackgroundStyle, `url('assets/backgrounds/${fileName}-low.webp')`);
		
		// After loading/fetching the _hqBackground's background image 
		this._hqBackground.onload = () => {

			// After downloading the HQ image, set it as bg of body
			this._styleBodyBackgrond(this._bodyStyle, `url('${this._hqBackground.src}')`);

			// Add a delay before hiding the overlay dummy background to avoid the white flicker
			setTimeout (
				() => {
					// Hide the dummy background
					this._dummyBodyBackground.classList.add('dummyBackgroundHide');
					
					// Remove class to unblur
					this._dummyBodyBackground.classList.remove('dummyBackgroundBlur');

				},
				3000
			);
		};

		// Add a delay when to fetch the hq background
		setTimeout (
			() => {
				this._hqBackground.src = `assets/backgrounds/${fileName}.webp`;
			},
			500
		);
	}
	
	_startLazyLoad = () => {
		const date = new Date();
		const hour = date.getHours();

		if (hour >= 6 && hour < 12) {
			this._lazyLoadBackground('morning');

		} else if (hour >= 12 && hour < 18 ) {
			this._lazyLoadBackground('noon');

		} else {
			this._lazyLoadBackground('evening');
		}
	}
}