javascript - 允许空值，同时在提交后保留输入值

Question

表单中有几个可选的输入字段。我想在提交后保留他们的输入值。

我正在阅读这篇文章JS input type date field, keep values selected after form submission但由于输入字段都是可选的，我收到错误消息:can not set property value of null in web console。

<form class="cat_select" onsubmit="setCate()">
    <input class="l2" name="l2" id="l2" type="text" style="width:30%">
    <input class="l3" name="l3" id="l3" type="text" style="width:50%">
    <button class="btn btn-outline-success my-2 my-sm-0" type="submit" id="cat_submit">Submit</button>
</form>

function setCate(){
    var l2 = document.getElementById('l2').value;
    var l3 = document.getElementById('l3').value;
}
function getl2(){
    if (localStorage.getItem("user_selected_l2") !== null) {
        return localStorage.getItem("user_selected_l2");
    }   
}
function getl3(){
    if (localStorage.getItem("user_selected_l3") !== null) {
        return localStorage.getItem("user_selected_l3");
    }   
}
//seems that can't set these as null
document.getElementById('l2').value=getl2(); 
document.getElementById('l3').value=getl3();

Answer 1

您可以使用条件三元将 null 替换为空字符串:

document.getElementById('l2').value = getl2()? getl2() : ''; 
document.getElementById('l3').value = getl3()? getl3() : '';

或者您可以使用相同的逻辑返回空字符串而不是 null:

function getl3(){
    if (localStorage.getItem("user_selected_l3") !== null) {
        return localStorage.getItem("user_selected_l3")?
               localStorage.getItem("user_selected_l3") : '';
    }   
}