How to get the actual width of an element in jQuery, even with border-box: box-sizing

This is ridiculous, but per the jQuery documentation:

Note that .width() will always return the content width, regardless of the value of the CSS box-sizing property. As of jQuery 1.8, this may require retrieving the CSS width plus box-sizing property and then subtracting any potential border and padding on each element when the element has box-sizing: border-box. To avoid this penalty, use .css( "width" ) rather than .width().

function parsePx(input) {
	let match;
	
	if (match = /^([0-9+])px$/.exec(input)) {
		return parseFloat(match[1]);
	} else {
		throw new Error("Value is not in pixels!");
	}
}

$.prototype.actualWidth = function() {
	/* WTF, jQuery? */
	let isBorderBox = (this.css("box-sizing") === "border-box");
	let width = this.width();
	
	if (isBorderBox) {
		width = width
			+ parsePx(this.css("padding-left"))
			+ parsePx(this.css("padding-right"))
			+ parsePx(this.css("border-left-width"))
			+ parsePx(this.css("border-right-width"));
	}
	
	return width;
}