var sign ;
function check(v){/*  调用函数时，传入this.value*/
	var fth_num = parseInt(num.value);

	switch (v) {
		case 'C':
			num.value = 0;
			break;	
		case  '+':
			sign = '+';
			num.value = 0;
			break;
		case  '-':
			sign = '-';
			num.value = 0;
			break;
		case  '*':
			sign = '*';
			num.value = 0;
			break;
		case  '/':
			sign = '/';
			num.value = 0;
			break;
		case '=':
			switch (sign) {
				case '+':
					console.log(num.value,fth_num);

					num.value = parseInt(num.value)+ fth_num;
					break;

				case '-':
					num.value -= fth_num;
					break;

				case '*':
					num.value *= fth_num;
					break;

				case '/':
					num.value /= fth_num;
					break;
				}


	}
}

我通过控制台打印的信息发现，每一次加法运算的两个数字，不管按哪个，都会变成两个 最后一次按的数字 进行相加