1.从表单获取的value是字符串，如果需要为数字相加，则需要转换为number类型

<input type="number" id="val1">
<input type="number" id="val2">
<button id="btn">获取值</button>
<p id="Result"></p>

<script>
    var input1=document.getElementById("val1");
    var input2=document.getElementById("val2");
    var Result=document.getElementById("Result");
    document.getElementById("btn").addEventListener("click", function () {
        var val1=input1.value;
        var val2=input2.value;
        Result.innerHTML=val1+val2//字符串的相加
    })
</script>
2.数字的相加，如没有确定类型，最好多加一步转换为number类型相加

localStorage.setItem("num1",1);
localStorage.setItem("num2",1);
console.log(localStorage.getItem("num1")+localStorage.getItem("num2"))//11;

console.log(Number(localStorage.getItem("num1"))+Number(localStorage.getItem("num2")))//2

js弱数据类型的坑