javascript - 如何添加相同参数中的字符串

Question

我想将两个字符串加在一起，这样如果单击7 和8，该函数将设置outputbox 的内部HTML到 78。但是，使用此当前功能，它仅显示最近选择的号码。我怎样才能让它显示已选择的每个数字？

function selectNum(num)
{
    num = num + num // What goes here?
    document.getElementById("outputbox").innerHTML = num;
}

<button onclick = "selectNum('7')">7</button>
<button onclick = "selectNum('8')">8</button>
<button onclick = "selectNum('9')">9</button>
<button onclick = "selectNum('4')">4</button>
<button onclick = "selectNum('5')">5</button>
<button onclick = "selectNum('6')">6</button>
<button onclick = "selectNum('1')">1</button>
<button onclick = "selectNum('2')">2</button>
<button onclick = "selectNum('3')">3</button>

Answer 1

尝试:

var number = '';
function selectNum(num) {
    number += num;
    document.getElementById("outputbox").innerHTML = number;
}