连接3个字符串 - 在线编辑器

          <p id="demo">单击按钮连接2个字符串形成一个新的字符串。</p>
<button onclick="myFunction()">点我</button>
<script>
  function myFunction() {
    var txt1 = "Hello ";
    var txt2 = "world!";
    var txt3 = " Have a nice day!";
    var n = txt1.concat(txt2, txt3);
    document.getElementById("demo").innerHTML = n;
  }
</script>
        