The Brave Browser removed "Brave" from its User-Agent in v0.9. DuckDuckGo can detect Brave & this promise utilizes their API to detect Brave.

Detect_Brave_Browser.js

const detectBraveBrowser = () => {
	return new Promise((resolve, reject) => {
		if(!navigator.userAgent.includes('Chrome')) { return resolve(false); }
    
		const xhr = new XMLHttpRequest();
		const onload = () => {
			if(xhr.status >= 200 && xhr.status < 300) {
				const response = JSON.parse(xhr.responseText);

				if(!response) { return resolve(false); }
				if(!response.Answer) { return resolve(false); }
				if(!response.Answer.includes('Brave')) { return resolve(false); }

				return resolve(true);
			} else {
				return reject(JSON.parse(xhr.responseText));
			}
		};

		xhr.onload = onload;
		xhr.open('GET', 'https://api.duckduckgo.com/?q=useragent&format=json');
		xhr.send();
	});
};


detectBraveBrowser().then((isBrave) => {
	console.log('isBrave', isBrave);
}).catch((error) => { console.error(error); });

I had the same problem. You just have to put everything in an async function, then it should work.