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<h1 class="entry-title">maxlength Constraint Validation Oddities in Major Browsers</h1>
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<time datetime="2012-10-17T12:48:00-04:00" pubdate data-updated="true">Oct 17<span>th</span>, 2012</time>
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<div class="entry-content"><p>The <code>maxlength</code> attribute has been around in browsers for a long time. When provided all browsers prevent the user entering a value that exceeds the <code>maxlength</code>.</p>
<p>For example you cannot type more than 2 characters in the textbox below:</p>
<p><input type="text" maxlength="2" /></p>
<h3>Constraint Validation</h3>
<p><a href="https://developer.mozilla.org/en-US/docs/HTML/HTML5/Constraint_validation_">Constraint validation</a> is an HTML5 spec that provides native client side form validation in the browser. As part of its <a href="https://developer.mozilla.org/en-US/docs/HTML/Forms_in_HTML#Constraint_Validation_API">API</a>, all <code><input></code> and <code><textarea></code> elements have a <code>validity.tooLong</code> property that is <code>true</code> when the length of the <code>value</code> exceeds the <code>maxlength</code>.</p>
<p>But if the browser prevents this then why does the property exist?</p>
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<h3>Prefilled value Attribute</h3>
<p>Assume that you’re filling the <code>value</code> of form elements with information from a database and you end up with something like this:</p>
<p><code><input type="text" maxlength="1" value="ABC" /></code></p>
<p>How will the browser handle this?</p>
<p>All browsers will prevent entry of additional characters, but they do not trim excess characters already present. Additionally all browsers will allow a form containing the above input to submit. <em>Note: Opera is the only browser to set the <code>validity.tooLong</code> property to <code>true</code> in this situation. Despite this, it does not prevent form submission.</em></p>
<p>Why is submission not prevented? The key is in the <a href="http://www.whatwg.org/specs/web-apps/current-work/#concept-input-value-dirty-flag">specification</a>:</p>
<blockquote><p>Constraint validation: If an element has a maximum allowed value length, its dirty value flag is true, its value was last changed by a user edit (as opposed to a change made by a script), and the code-unit length of the element’s value is greater than the element’s maximum allowed value length, then the element is suffering from being too long.</p></blockquote>
<p>The <a href="http://www.whatwg.org/specs/web-apps/current-work/#concept-input-value-dirty-flag">dirty flag</a> essentially means the the user has changed the value of an element. Therefore, in order to be <code>tooLong</code> the element must have been last interacted with by a user edit.</p>
<h3>Actually Triggering tooLong</h3>
<p>So let’s take another approach. What happens if you have the same input:</p>
<p><code><input type="text" maxlength="1" value="ABC" /></code></p>
<p>…remove one character, then submit? You can try it for yourself below:</p>
<iframe style="width: 100%; height: 180px;" src="http://jsfiddle.net/tj_vantoll/epCQe/embedded/result,html/" allowfullscreen="allowfullscreen" frameborder="0"></iframe>
<p>Upon edit IE10 and Chrome will set the <code>validity.tooLong</code> property to <code>true</code> and prevent form submission. If the user attempts to submit a form after removing the “C” they will see the following in those browsers:</p>
<p><img src="/images/posts/2012-10-17/Chrome.png" title="Chrome" alt="Chrome" />
<img src="/images/posts/2012-10-17/IE10.png" title="IE10" alt="IE10" /></p>
<p>Firefox, Safari, and Opera incorrectly handle this situation and allow the form to be submitted anyways.</p>
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<span class="byline author vcard">Posted by <span class="fn">TJ VanToll</span></span>
<time datetime="2012-10-17T12:48:00-04:00" pubdate data-updated="true">Oct 17<span>th</span>, 2012</time>
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