<script>
		function Counter (a, b, c) {
			var sum = 0;
			if 		(c == 1) { sum = a + b; }
			else if (c == 2) { sum = a - b; }
			else if (c == 3) { sum = a * b; }
			else if (c == 4) { sum = a / b; }
			return sum;
		}
		var pop = parseInt(prompt(
			'1.加法运算\n2.减法运算\n3.乘法运算\n4.除法运算\n5.退出\n请输入您的选项：'
		));

		while (pop != 5) {
			var a = parseInt(prompt('请输入第一个数字'));
			var b = parseInt(prompt('请输入第二个数字'));
			var counter = Counter(a, b, pop);
			alert(counter);
			pop = prompt(
				'1.加法运算\n2.减法运算\n3.乘法运算\n4.除法运算\n5.退出\n请输入您的选项：'
			)
		}
	</script>