How to Set Html5 Required Attribute in JavaScript

How to set HTML5 required attribute in Javascript?

Short version

element.setAttribute("required", "");    //turns required on
element.required = true;                 //turns required on through reflected attribute
jQuery(element).attr('required', '');    //turns required on
$("#elementId").attr('required', '');    //turns required on

element.removeAttribute("required");     //turns required off
element.required = false;                //turns required off through reflected attribute
jQuery(element).removeAttr('required');  //turns required off
$("#elementId").removeAttr('required');  //turns required off

if (edName.hasAttribute("required")) { }  //check if required
if (edName.required) { }                 //check if required using reflected attribute

Long Version

Once T.J. Crowder managed to point out reflected properties, i learned that following syntax is wrong:

element.attributes["name"] = value; //bad! Overwrites the HtmlAttribute object
element.attributes.name = value;    //bad! Overwrites the HtmlAttribute object
value = element.attributes.name;    //bad! Returns the HtmlAttribute object, not its value
value = element.attributes["name"]; //bad! Returns the HtmlAttribute object, not its value

You must go through element.getAttribute and element.setAttribute:

element.getAttribute("foo");         //correct
element.setAttribute("foo", "test"); //correct

This is because the attribute actually contains a special HtmlAttribute object:

element.attributes["foo"];           //returns HtmlAttribute object, not the value of the attribute
element.attributes.foo;              //returns HtmlAttribute object, not the value of the attribute

By setting an attribute value to "true", you are mistakenly setting it to a String object, rather than the HtmlAttribute object it requires:

element.attributes["foo"] = "true";  //error because "true" is not a HtmlAttribute object
element.setAttribute("foo", "true"); //error because "true" is not an HtmlAttribute object

Conceptually the correct idea (expressed in a typed language), is:

HtmlAttribute attribute = new HtmlAttribute();
attribute.value = "";
element.attributes["required"] = attribute;

This is why:

• getAttribute(name)
• setAttribute(name, value)

exist. They do the work on assigning the value to the HtmlAttribute object inside.

On top of this, some attribute are reflected. This means that you can access them more nicely from Javascript:

//Set the required attribute
//element.setAttribute("required", ""); 
element.required = true;

//Check the attribute
//if (element.getAttribute("required")) {...}
if (element.required) {...}

//Remove the required attribute
//element.removeAttribute("required");
element.required = false;

What you don't want to do is mistakenly use the .attributes collection:

element.attributes.required = true;     //WRONG!
if (element.attributes.required) {...}  //WRONG!
element.attributes.required = false;    //WRONG!

Testing Cases

This led to testing around the use of a required attribute, comparing the values returned through the attribute, and the reflected property

document.getElementById("name").required;
document.getElementById("name").getAttribute("required");

with results:

HTML                         .required        .getAttribute("required")
==========================   ===============  =========================
<input>                      false (Boolean)  null (Object)
<input required>             true  (Boolean)  "" (String)
<input required="">          true  (Boolean)  "" (String)
<input required="required">  true  (Boolean)  "required" (String)
<input required="true">      true  (Boolean)  "true" (String)
<input required="false">     true  (Boolean)  "false" (String)
<input required="0">         true  (Boolean)  "0" (String)

Trying to access the .attributes collection directly is wrong. It returns the object that represents the DOM attribute:

edName.attributes["required"] => [object Attr]
edName.attributes.required    => [object Attr]

This explains why you should never talk to the .attributes collect directly. You're not manipulating the values of the attributes, but the objects that represent the attributes themselves.

How to set required?

What's the correct way to set required on an attribute? You have two choices, either the reflected property, or through correctly setting the attribute:

element.setAttribute("required", "");         //Correct
edName.required = true;                       //Correct

Strictly speaking, any other value will "set" the attribute. But the definition of Boolean attributes dictate that it should only be set to the empty string "" to indicate true. The following methods all work to set the required Boolean attribute,

but do not use them:

element.setAttribute("required", "required"); //valid, but not preferred
element.setAttribute("required", "foo");      //works, but silly
element.setAttribute("required", "true");     //Works, but don't do it, because:
element.setAttribute("required", "false");    //also sets required boolean to true
element.setAttribute("required", false);      //also sets required boolean to true
element.setAttribute("required", 0);          //also sets required boolean to true

We already learned that trying to set the attribute directly is wrong:

edName.attributes["required"] = true;       //wrong
edName.attributes["required"] = "";         //wrong
edName.attributes["required"] = "required"; //wrong
edName.attributes.required = true;          //wrong
edName.attributes.required = "";            //wrong
edName.attributes.required = "required";    //wrong

How to clear required?

The trick when trying to remove the required attribute is that it's easy to accidentally turn it on:

edName.removeAttribute("required");     //Correct
edName.required = false;                //Correct

With the invalid ways:

edName.setAttribute("required", null);    //WRONG! Actually turns required on!
edName.setAttribute("required", "");      //WRONG! Actually turns required on!
edName.setAttribute("required", "false"); //WRONG! Actually turns required on!
edName.setAttribute("required", false);   //WRONG! Actually turns required on!
edName.setAttribute("required", 0);       //WRONG! Actually turns required on!

When using the reflected .required property, you can also use any "falsey" values to turn it off, and truthy values to turn it on. But just stick to true and false for clarity.

How to check for required?

Check for the presence of the attribute through the .hasAttribute("required") method:

if (edName.hasAttribute("required"))
{
}

You can also check it through the Boolean reflected .required property:

if (edName.required)
{
}

HTML5 form required attribute. Set custom validation message?

Use setCustomValidity:

document.addEventListener("DOMContentLoaded", function() {
    var elements = document.getElementsByTagName("INPUT");
    for (var i = 0; i < elements.length; i++) {
        elements[i].oninvalid = function(e) {
            e.target.setCustomValidity("");
            if (!e.target.validity.valid) {
                e.target.setCustomValidity("This field cannot be left blank");
            }
        };
        elements[i].oninput = function(e) {
            e.target.setCustomValidity("");
        };
    }
})

I changed to vanilla JavaScript from Mootools as suggested by @itpastorn in the comments, but you should be able to work out the Mootools equivalent if necessary.

Edit

I've updated the code here as setCustomValidity works slightly differently to what I understood when I originally answered. If setCustomValidity is set to anything other than the empty string it will cause the field to be considered invalid; therefore you must clear it before testing validity, you can't just set it and forget.

Further edit

As pointed out in @thomasvdb's comment below, you need to clear the custom validity in some event outside of invalid otherwise there may be an extra pass through the oninvalid handler to clear it.

HTML5 required attribute one of two fields

Update 2020-06-21 (ES6):

Given that jQuery has become somewhat unfashionable in the JavaScript world and that ES6 provides some nice syntactic sugar, I have written a pure JS equivalent to the original answer:

document.addEventListener('DOMContentLoaded', function() {
  const inputs = Array.from(
    document.querySelectorAll('input[name=telephone], input[name=mobile]')
  );

  const inputListener = e => {
    inputs
      .filter(i => i !== e.target)
      .forEach(i => (i.required = !e.target.value.length));
  };

  inputs.forEach(i => i.addEventListener('input', inputListener));
});

<form method="post">
  Telephone:
  <input type="tel" name="telephone" value="" required>
  <br>Mobile:
  <input type="tel" name="mobile" value="" required>
  <br>
  <input type="submit" value="Submit">
</form>

document.getElementById("website").removeAttribute("required");

document.getElementById("website").required = false;

<input type=submit formnovalidate name=cancel value="Cancel">

window.addEventListener("DOMContentLoaded", () => {
  const sel = document.querySelector("[name=InquiryType]"),
    terms = document.getElementById("terms_target"),
    form = document.getElementById("myForm"),
    agree = form.checked; // checked is a poor name for a field
  toggleAgree = () => {
    const show = ["1", "2"].includes(sel.value);
    terms.hidden = !show;
    agree.required = show;
  };
  sel.addEventListener("change", toggleAgree)
  toggleAgree(); // initialise
})

<form action="process_affiliate.php" id="myForm" method="post">
  <select name="InquiryType" required>
    <option value="">Please Select Your Inquiry Type</option>
    <option value="1">Option 1</option>
    <option value="2">Option 2</option>
    <option value="3">Option 3</option>
  </select>
  <div id="terms_target" hidden>
    <input type="checkbox" name="checked" value="Checked" required="" />I agree to the terms
  </div>
  <input type="submit" value="Submit">
</form>