JavaScript中字符串运算符是什么？有哪些？

<p id="add">+ 号运算符用于连接字符串</p>
<button onclick="string()">clickMe</button>

<script>
function string() {
    var str1 = "Welcome ";
    var str2 = "to ";
    var str3 = "xkd!"
    var strs = str1 + str2 + str3;
    document.getElementById("add").innerHTML = strs;
}
</script>

<p id="equal">+= 号运算符用于连接字符串并赋给第一个值</p>
<button onclick="string2()">clickMe2</button>

<script>
function string2() {
    var str2_1 = "New ";
    var str2_2 = "version";
    str2_1 += str2_2;
    document.getElementById("equal").innerHTML = str2_1;
}
</script>