使用JavaScript / HTML进行基本表单验证

Question

I'm writing a form validation script for my Contact Us form I made. The script is pretty straight forward, I am wondering why it isn't working correctly.

No matter what fields I have content in, it always says that field is empty after running the script.

Here is my code:

    var firstName = document.getElementById("fname");
var lastName = document.getElementById("lname");
var email = document.getElementById("email");
var message = document.getElementById("msg");
var errors = "";

function formValidation() {
    if (firstName==="" || firstName=== null)
    {
        errors += "-The First Name field is blank! \n";
    }   
    if (lastName==="" || lastName=== null)
    {
        errors += "-The Last Name field is blank! \n";

    }   
    if (email==="" || email=== null)
    {
        errors += "-The E-mail Address field is blank! \n";

    }
    if (message==="" || message=== null)
    {
        errors += "-The Message field is blank! \n";

    }

    if (errors !== "") {
        alert("Whoops! \n \n" + errors);    
    }
    if (errors === "") {
        alert("Message Sent!");
    }

}

Additionally, here is the jsfiddle I made: http://jsfiddle.net/3DxZj/1/

Thank you.

Answer 1

First, you are trying to get the elements by their ids before they exist in the DOM (the script is above the form).

Second, if you corrected that then you would be comparing the HTMLInputElements themselves to an empty string, instead of their .value properties.

Third, you never reset errors so if anybody did get an error and them fixed it, they would still get the error alert when they tried again.

Add .value to the elements you are trying to get and move the following code so it is inside the function.

var firstName = document.getElementById("fname").value;
var lastName = document.getElementById("lname").value;
var email = document.getElementById("email").value;
var message = document.getElementById("msg").value;
var errors = "";

You are also only checking for errors when the form is submitted using the submit button. You should do this when the form is submitted instead.

Move the onclick attribute contents to an onsubmit attribute on the form element. Better yet, bind your event listener with JS .

You aren't preventing the normal action of the form when there are errors. Presumably you want it to stop the data from submitting. Either:

1. Use addEventListener (see above), accept an argument for your function and call .preventDefault() on that argument's value when there are errors or
2. Add return to the front of your onsubmit attribute value and return false from the function when there are errors.

Also note that

• Your label elements are useless ; they need for attributes.
• You shouldn't use tables to layout (most) forms .
• The value s will always be strings so there is no point in comparing to null .

Answer 2

You are querying the dom elements but not their values. The correct way would be

var firstName = document.getElementById("fname");
var lastName = document.getElementById("lname");
var email = document.getElementById("email");
var message = document.getElementById("msg");
var errors = "";

function formValidation() {
    if (firstName.value==="" || firstName.value=== null)
    {
        errors += "-The First Name field is blank! \n";
    }   
    if (lastName.value==="" || lastName.value=== null)
    {
        errors += "-The Last Name field is blank! \n";

    }   
    if (email.value==="" || email.value=== null)
    {
        errors += "-The E-mail Address field is blank! \n";

    }
    if (message.value==="" || message.value=== null)
    {
        errors += "-The Message field is blank! \n";

    }

    if (errors !== "") {
        alert("Whoops! \n \n" + errors);    
    }
    if (errors === "") {
        alert("Message Sent!");
    }

}

EDIT: Stupid me, didn't check the jsfiddle so I solved only one of your problems while making a mistake in my solution (corrected now), so stick to Quentins answer

Answer 3

The issue is that you are not returning the .value of the form fields.

eg: var firstName = document.getElementById("fname").value;

Also, you should declare your vars inside the function.

Try this:

function formValidation() {
var firstName = document.getElementById("fname").value;
var lastName = document.getElementById("lname").value;
var email = document.getElementById("email").value;
var message = document.getElementById("msg").value;
var errors = "";
if (firstName==="" || firstName=== null)
{
    errors += "-The First Name field is blank! \n";
}   
if (lastName==="" || lastName=== null)
{
    errors += "-The Last Name field is blank! \n";

}   
if (email==="" || email=== null)
{
    errors += "-The E-mail Address field is blank! \n";

}
if (message==="" || message=== null)
{
    errors += "-The Message field is blank! \n";

}

if (errors !== "") {
    alert("Whoops! \n \n" + errors);    
}
if (errors === "") {
    alert("Message Sent!");
}

}