javascript - 我想知道如何在 javascript 中进行表单验证

Question

我正在尝试学习表单验证，但它不起作用。

// birthyear must be between 1900 and 2012. turn the birth year textbox color to yellow
        window.onload = function() {
        document.forms[0].onsubmit = function() {
        for(i = 0; i < document.forms[0].elements.length; i++){

       var x =
       document.forms[birthyear].value

    if (x != (>1900 &&<=2012)){
    alert("Must be between 1900 and 2012");
    x.this.style.color ="yellow";
    return false;

//这就是我创建表单的方式:

<form action = "fake.php"></br>

    Username  
   <input class ="required"type = "text" name = "username" id ="username" /><br>
   Username
   <input class = "required"type = "text" name ="username"id ="username"/>   <br>


Birthyear
<input class  = "required" type = "number" name = "birthyear" id= "birthyear"/>

<input type = "submit"/>
</form>

Answer 1

if(x<1900 || x> 2012){
    alert("invalid year");

使用这样的 if 语句并尝试

并检查变量 x 是否采用用户正确输入的值。只需对 x 变量发出警报并首先确认