具有html表单验证的Javascript不起作用

Question

i'm trying to validate a form using a simple javascript script however it is not working, can anyone tell me what is wrong with my code? (sorry for my vague question)

JS code:

<script type="text/javascript">
function validation()
{                                 
    var fName=document.forms["teacherReg"]["firstname"].value;
    var lName=document.forms["teacherReg"]["lastname"].value;
    var uName=document.forms["teacherReg"]["username"].value;
    var pWord=document.forms["teacherReg"]["password"].value;
    var cPWord=document.forms["teacherReg"]["confirmpassword"].value;

    if (fName="" || lName="" || uName="" || pWord="")
    {
        alert("Not a valid entry");
        return false;
    }

}
</script>

html form:

<form name="teacherReg" action="" method="POST" onsubmit="return validation();">
    1. First name:
    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
    2. Last name:<br/><input type="text" name="firstname" id="firstname" />
    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
    <input type="text" name="lastname" id="lastname" /><br/><br/>                             
    3. Desired Username:
    <br/><input type="text" name="username" id="username" /><br/><br/>
    4. Desired Password:
    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
    5. Confirm Password:<br/><input type="password" name="password" id="password" />
    &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
    <input type="password" name="confirmpassword" id="confirmpassword" /> <br/><br/>
    <center><input type="submit" value="Register" name="submitbutton" class="button" /></center>
</form>

I expect it to return false if any of the fields "fName, lName, uName, pWord" are blank, however it is always returning true

Answer 1

The problem is that you confused = (assignment operator) operator with == (comparison operator):

if (fName = "" || lName = "" || uName = "" || pWord = "") {

It should be

if (fName == "" || lName == "" || uName == "" || pWord == "") {

Fixed demo: http://jsfiddle.net/49xDH/

Answer 2

这就是问题：

 if (fName=="" || lName=="" || uName=="" || pWord=="")

Answer 3

= is assignment operator , where == is comparison operator. use == for comparing values.

Answer 4

使用比较运算符==代替赋值运算符= 。

Answer 5

Please try to use the ID instead of the form object.

function validation(){
    var username = document.getElementById('username').value;
    // etc.
}