JSFiddle - React, Tailwind, and code Playground

HTML

<html>
	<head>
	</head>
	<body>
		<script>
			var bestResult = '';
			var bestResultCount = 0;
			var bestResultStartInteger = 0;

			// I made an assumption here, that only odd numbers are of interest
			// reason being, that, given task conditions, any even number will
			// generate a smaller number, than the same number minus 1
			// e.g.: 502 -> 251 -> 754 but 501 -> 1504
			// not mentioning even numbers which produce more even numbers when divided by 2
			// like 500 -> 250 -> 125 -> 376
			// and we want as big numbers as possible, to make the string longer
			// hence starting from the top of the "stack"
			for (var i = 999999; i > 1; i = i - 2) {
				let tmp = i;
				let currentResult = i;
				let currentResultCount = 0;

				while (tmp > 1) {
					if (tmp % 2 === 0) { // even
						tmp = tmp / 2;
					} else {
						tmp = (3 * tmp) + 1;
					}

					currentResult = currentResult +'-'+ tmp;
					currentResultCount = currentResultCount + 1;
				}

				if (currentResultCount > bestResultCount) {
					bestResult = currentResult;
					bestResultCount = currentResultCount;
					bestResultStartInteger = i;
				}
			}

			console.log(bestResult);
			console.log(bestResultCount);
			console.log(bestResultStartInteger);
			
			alert('Answer: '+ bestResultStartInteger);
		</script>
  </body>
</html>