Constraint validation

The creation of web forms has always been a complex task. While marking up the form itself is easy, checking whether each field has a valid and coherent value is even more difficult, and informing the user about the problem may become a headache. HTML5 introduced new mechanisms for forms: it added new semantic types for the <input> element and constraint validation to ease the work of checking the form content on the client side. Basic, usual constraints can be checked, without the need for JavaScript, by setting new attributes; more complex constraints can be tested using the HTML5 Constraint Validation API.

Intrinsic and basic constraints

In HTML5, basic constraints are declared in two ways:

• By choosing the most semantically appropriate value for the type attribute of the <input> element, e.g., choosing the email type automatically creates a constraint that checks whether the value is a valid e-mail address.
• By setting values on validation-related attributes, allowing basic constraints to be described in a simple way, without the need for JavaScript.

Semantic input types

The intrinsic constraints for the type attribute are:

Note that most input types don't have intrinsic constraints, as some are simply barred from constraint validation or have a sanitization algorithm transforming incorrect values to a correct default. 

Validation-related attributes

The following attributes are used to describe basic constraints:

Constraint validation process

Constraint validation is done through the Constraint Validation API either on a single form element or at the form level, on the <form> element itself. The constraint validation is done in the following ways:

• By a call to the checkValidity() method of a form-related DOM interface (HTMLInputElement, HTMLSelectElement, HTMLButtonElement or HTMLTextAreaElement), which evaluates the constraints only on this element, allowing a script to get this information. (This is typically done by the user-agent when determining which of the CSS pseudo-classes, :valid or :invalid, applies.)
• By a call to the checkValidity() function on the HTMLFormElement interface, which is called statically validating the constraints.
• By submitting the form itself, which is called interactively validating the constraints.

Complex constraints using HTML5 Constraint API

Using JavaScript and the Constraint API, it is possible to implement more complex constraints, for example, constraints combining several fields, or constraints involving complex calculations.

Basically, the idea is to trigger JavaScript on some form field event (like onchange) to calculate whether the constraint is violated, and then to use the method field.setCustomValidity() to set the result of the validation: an empty string means the constraint is satisfied, and any other string means there is an error and this string is the error message to display to the user.

Constraint combining several fields: Postal code validation

The postal code format varies from one country to another. Not only do most countries allow an optional prefix with the country code (like D- in Germany, F- in France or Switzerland), but some countries have postal codes with only a fixed number of digits; others, like the UK, have more complex structures, allowing letters at some specific positions.

As an example, we will add a script checking the constraint validation for this simple form:

<form>
    <label for="ZIP">ZIP : </label>
    <input type="text" id="ZIP"> 
    <label for="Country">Country : </label>
    <select id="Country">
      <option value="ch">Switzerland</option>
      <option value="fr">France</option>
      <option value="de">Germany</option>
      <option value="nl">The Netherlands</option>
    </select>
    <input type="submit" value="Validate">
</form>

This displays the following form: 

Postal Code: Country: SwitzerlandFranceGermanyThe Netherlands

First, we write a function checking the constraint itself:

function checkZIP() {
  // For each country, defines the pattern that the ZIP has to follow
  var constraints = {
    ch : [ '^(CH-)?\\d{4}$', "Switzerland ZIPs must have exactly 4 digits: e.g. CH-1950 or 1950" ],
    fr : [ '^(F-)?\\d{5}$' , "France ZIPs must have exactly 5 digits: e.g. F-75012 or 75012" ],
    de : [ '^(D-)?\\d{5}$' , "Germany ZIPs must have exactly 5 digits: e.g. D-12345 or 12345" ],
    nl : [ '^(NL-)?\\d{4}\\s*([A-RT-Z][A-Z]|S[BCE-RT-Z])$',
                    "Nederland ZIPs must have exactly 4 digits, followed by 2 letters except SA, SD and SS" ]
  };
  
  // Read the country id
  var country = document.getElementById("Country").value;

  // Get the NPA field
  var ZIPField = document.getElementById("ZIP");

  // Build the constraint checker
  var constraint = new RegExp(constraints[country][0], "");
    console.log(constraint);


  // Check it!
  if (constraint.test(ZIPField.value)) {
    // The ZIP follows the constraint, we use the ConstraintAPI to tell it
    ZIPField.setCustomValidity("");
  }
  else {
    // The ZIP doesn't follow the constraint, we use the ConstraintAPI to
    // give a message about the format required for this country
    ZIPField.setCustomValidity(constraints[country][1]);
  }
}

Then we link it to the onchange event for the <select> and the oninput event for the <input>:

window.onload = function () {
    document.getElementById("Country").onchange = checkZIP;
    document.getElementById("ZIP").oninput = checkZIP;
}

You can see a live example of the postal code validation.  

Limiting the size of a file before its upload

Another common constraint is to limit the size of a file to be uploaded. Checking this on the client side before the file is transmitted to the server requires combining the Constraint API, and especially the field.setCustomValidityUI() method, with another JavaScript API, here the HTML5 File API.

Here is the HTML part:

<label for="FS">Select a file smaller than 75 kB : </label>
<input type="file" id="FS">

This displays:

Select a file smaller than 75 kB :

The JavaScript reads the file selected, uses the File.size() method to get its size, compares it to the (hard coded) limit, and calls the Constraint API to inform the browser if there is a violation:

function checkFileSize() {
  var FS = document.getElementById("FS");
  var files = FS.files;

  // If there is (at least) one file selected
  if (files.length > 0) {
     if (files[0].size > 75 * 1024) { // Check the constraint
       FS.setCustomValidity("The selected file must not be larger than 75 kB");
       return;
     }
  }
  // No custom constraint violation
  FS.setCustomValidity("");
}

Finally we hook the method with the correct event:

window.onload = function () {
  document.getElementById("FS").onchange = checkFileSize;
}

You can see a live example of the File size constraint validation.

Visual styling of constraint validation

Apart from setting constraints, web developers want to control what messages are displayed to the users and how they are styled.

Controlling the look of elements

The look of elements can be controlled via CSS pseudo-classes.

:required and :optional CSS pseudo-classes

The :required and :optional pseudo-classes allow writing selectors that match form elements that have the required attribute, or that don't have it.

:-moz-placeholder CSS pseudo-class

See :-moz-placeholder.

:valid :invalid CSS pseudo-classes

The :valid and :invalid pseudo-classes are used to represent <input> elements whose content validates and fails to validate respectively according to the input's type setting. These classes allow the user to style valid or invalid form elements to make it easier to identify elements that are either formatted correctly or incorrectly.

Default styles

Controlling the text of constraints violation

The x-moz-errormessage attribute

The x-moz-errormessage attribute is a Mozilla extension that allows you to specify the error message to display when a field does not successfully validate.

Constraint API's element.setCustomValidity()

The element.setCustomValidity(error) method is used to set a custom error message to be displayed when a form is submitted. The method works by taking a string parameter error. If error is a non-empty string, the method assumes validation was unsuccessful and displays error as an error message. If error is an empty string, the element is considered validated and resets the error message.

Constraint API's ValidityState object

The DOM ValidityState interface represents the validity states that an element can be in, with respect to constraint validation. Together, they help explain why an element's value fails to validate, if it's not valid.

Examples of personalized styling

HTML4 fallback

Trivial fallback

JS fallback

Conclusion