amp-form

<script async custom-element="amp-form" src="https://cdn.ampproject.org/v0/amp-form-0.1.js"></script>

Usage

The amp-form extension allows you to create forms (<form>) to submit input fields in an AMP document. The amp-form extension also provides polyfills for some missing behaviors in browsers.

Before creating a <form>, you must include the required script for the <amp-form> extension, otherwise your document will be invalid. If you're using input tags for purposes other than submitting their values (e.g., inputs not inside a <form>), you do not need to load the amp-form extension.

<form method="post"
    action-xhr="https://example.com/subscribe"    target="_top">
    <fieldset>
      <label>
        <span>Name:</span>
        <input type="text"
          name="name"
          required>
      </label>
      <br>
      <label>
        <span>Email:</span>
        <input type="email"
          name="email"
          required>
      </label>
      <br>
      <input type="submit"
        value="Subscribe">
    </fieldset>
    <div submit-success>
      <template type="amp-mustache">
        Subscription successful!
      </template>
    </div>
    <div submit-error>
      <template type="amp-mustache">
        Subscription failed!
      </template>
    </div>
  </form>

Inputs and fields

Allowed

• Other form-related elements, including: <textarea>, <select>, <option>, <fieldset>, <label>, <input type=text>, <input type=submit>, and so on.
• <input type=password> and <input type=file> inside of <form method=POST action-xhr>.
• amp-selector

Not Allowed

• <input type=button>, <input type=image>
• Most of the form-related attributes on inputs including: form, formaction, formtarget, formmethod and others.

(Relaxing some of these rules might be reconsidered in the future - please let us know if you require these and provide use cases).

For details on valid inputs and fields, see amp-form rules in the AMP validator specification.

Success and error response rendering

You can render success or error responses in your form by using amp-mustache, or success responses through data binding with amp-bind and the following response attributes:

To render responses with templating:

• Apply a response attribute to any descendant of the <form> element.
• Render the response in the child element by including a template via <template></template> or <script type="text/plain"></script> tag inside it or by referencing a template with a template="id_of_other_template" attribute.
• Provide a valid JSON object for responses to submit-success and submit-error. Both success and error responses should have a Content-Type: application/json header.

In the following example, the responses are rendered in an inline template inside the form.

<form ...>
  <fieldset>
    <input type="text" name="firstName" />
    ...
  </fieldset>
  <div submitting>
    <template type="amp-mustache">
      Form submitting... Thank you for waiting {{name}}.
    </template>
  </div>
  <div submit-success>
    <template type="amp-mustache">
      Success! Thanks {{name}} for subscribing! Please make sure to check your
      email {{email}} to confirm! After that we'll start sending you weekly
      articles on {{#interests}}<b>{{name}}</b> {{/interests}}.
    </template>
  </div>
  <div submit-error>
    <template type="amp-mustache">
      Oops! {{name}}, {{message}}.
    </template>
  </div>
</form>

The publisher's action-xhr endpoint returns the following JSON responses:

On success:

{
  "name": "Jane Miller",
  "interests": [
    {"name": "Basketball"},
    {"name": "Swimming"},
    {"name": "Reading"}
  ],
  "email": "email@example.com"
}

On error:

{
  "name": "Jane Miller",
  "message": "The email (email@example.com) you used is already subscribed."
}

You can render the responses in a referenced template defined earlier in the document by using the template's id as the value of the template attribute, set on the elements with the submit-success and submit-error attributes.

<template type="amp-mustache" id="submit_success_template">
  Success! Thanks {{name}} for subscribing! Please make sure to check your email
{{email}} to confirm! After that we'll start sending you weekly articles on
{{#interests}}<b>{{name}}</b> {{/interests}}.
</template>
<template type="amp-mustache" id="submit_error_template">
  Oops! {{name}}, {{message}}.
</template>

<form ...>
  <fieldset>
    ...
  </fieldset>
  <div submit-success template="submit_success_template"></div>
  <div submit-error template="submit_error_template"></div>
</form>

See the full example here.

To render a successful response with data binding

• Use the on attribute to bind the form submit-success attribute to AMP.setState().
• Use the event property to capture the response data.
• Add the state attribute to the desired element to bind the form response.

The following example demonstrates a form submit-success response with amp-bind:

<p [text]="'Thanks, ' + subscribe +'! You have successfully subscribed.'">
  Subscribe to our newsletter
</p>
<form
  method="post"
  action-xhr="/components/amp-form/submit-form-input-text-xhr"
  target="_top"
  on="submit-success: AMP.setState({'subscribe': event.response.name})"
>
  <div>
    <input type="text" name="name" placeholder="Name..." required />
    <input type="email" name="email" placeholder="Email..." required />
  </div>
  <input type="submit" value="Subscribe" />
</form>

When the form is submitted successfully it will return a JSON response similar to the following:

{
  "name": "Jane Miller",
  "email": "email@example.com"
}

Then amp-bind updates the <p> element's text to match the subscibe state:

...
<p [text]="'Thanks, ' + subscribe +'! You have successfully subscribed.'">
  Thanks Jane Miller! You have successfully subscribed.
</p>
...

Redirecting after a submission

You can redirect users to a new page after a successful form submission by setting the AMP-Redirect-To response header and specifying a redirect URL. The redirect URL must be a HTTPS URL, otherwise AMP will throw an error and redirection won't occur. HTTP response headers are configured via your server.

Make sure to update your Access-Control-Expose-Headers response header to include AMP-Redirect-To to the list of allowed headers. Learn more about these headers in CORS Security in AMP.

Example response headers:

AMP-Redirect-To: https://example.com/forms/thank-you
Access-Control-Expose-Headers: AMP-Redirect-To

Custom validations

The amp-form extension allows you to build your own custom validation UI by using the custom-validation-reporting attribute along with one the following reporting strategies: show-first-on-submit, show-all-on-submit or as-you-go.

To specify custom validation on your form:

1. Set the custom-validation-reporting attribute on your form to one of the validation reporting strategies.
2. Provide your own validation UI marked up with special attributes. AMP will discover the special attributes and report them at the right time depending on the reporting strategy you specified.

Here's an example:

<form method="post"
    action-xhr="https://example.com/subscribe"
    custom-validation-reporting="show-all-on-submit"    target="_blank">
    <fieldset>
      <label>
        <span>Name:</span>
        <input type="text"
          name="name"
          id="name5"
          required
          pattern="\w+\s\w+">
        <span visible-when-invalid="valueMissing"
          validation-for="name5"></span>
        <span visible-when-invalid="patternMismatch"
          validation-for="name5">
          Please enter your first and last name separated by a space (e.g. Jane Miller)
        </span>
      </label>
      <br>
      <label>
        <span>Email:</span>
        <input type="email"
          name="email"
          id="email5"
          required>
        <span visible-when-invalid="valueMissing"
          validation-for="email5"></span>
        <span visible-when-invalid="typeMismatch"
          validation-for="email5"></span>
      </label>
      <br>
      <input type="submit"
        value="Subscribe">
    </fieldset>
  </form>

For validation messages, if your element contains no text content inside, AMP will fill it out with the browser's default validation message. In the example above, when the name5 input is empty and validation is kicked off (i.e., user tried to submit the form) AMP will fill <span visible-when-invalid="valueMissing" validation-for="name5"></span> with the browser's validation message and show that span to the user.

Reporting strategies

Specify one of the following reporting options for the custom-validation-reporting attribute:

Show First on Submit

The show-first-on-submit reporting option mimics the browser's default behavior when default validation kicks in. It shows the first validation error it finds and stops there.

Show All on Submit

The show-all-on-submit reporting option shows all validation errors on all invalid inputs when the form is submitted. This is useful if you'd like to show a summary of validations.

As You Go

The as-you-go reporting option allows your user to see validation messages as they're interacting with the input. For example, if the user types an invalid email address, the user will see the error right away. Once they correct the value, the error goes away.

Interact and Submit

The interact-and-submit reporting option combines the behavior of show-all-on-submit and as-you-go. Individual fields will show any errors immediately after interactions, and on submit the form will show errors on all invalid fields.

Verification

HTML5 validation gives feedback based only on information available on the page, such as if a value matches a certain pattern. With amp-form verification you can give the user feedback that HTML5 validation alone cannot. For example, a form can use verification to check if an email address has already been registered. Another use-case is verifying that a city field and a zip code field match each other.

Here's an example:

<h4>Verification example</h4>
<form
  method="post"
  action-xhr="/form/verify-json/post"
  verify-xhr="/form/verify-json/post"  target="_blank">
    <fieldset>
        <label>
            <span>Email</span>
            <input type="text" name="email" required>
        </label>
        <label>
            <span>Zip Code</span>
            <input type="tel" name="zip" required pattern="[0-9]{5}(-[0-9]{4})?">
        </label>
        <label>
            <span>City</span>
            <input type="text" name="city" required>
        </label>
        <label>
            <span>Document</span>
            <input type="file" name="document" no-verify>
        </label>
        <div class="spinner"></div>
        <input type="submit" value="Submit">
    </fieldset>
    <div submit-success>
        <template type="amp-mustache">
            <p>Congratulations! You are registered with {{email}}</p>
        </template>
    </div>
    <div submit-error>
        <template type="amp-mustache">
{{#verifyErrors}}                <p>{{message}}</p>
{{/verifyErrors}}{{^verifyErrors}}                <p>Something went wrong. Try again later?</p>
{{/verifyErrors}}        </template>
    </div>
</form>

The form sends a __amp_form_verify field as part of the form data as a hint to the server that the request is a verify request and not a formal submit. This is helpful so the server knows not to store the verify request if the same endpoint is used for verification and for submit.

Here is how an error response should look for verification:

{
  "verifyErrors": [
    {"name": "email", "message": "That email is already taken."},
    {"name": "zip", "message": "The city and zip do not match."}
  ]
}

To remove a field from the verify-xhr request, add the no-verify attribute to the input element.

For more examples, see examples/forms.amp.html.

Variable substitutions

The amp-form extension allows platform variable substitutions for inputs that are hidden and that have the data-amp-replace attribute. On each form submission, amp-form finds all input[type=hidden][data-amp-replace] inside the form and applies variable substitutions to its value attribute and replaces it with the result of the substitution.

You must provide the variables you are using for each substitution on each input by specifying a space-separated string of the variables used in data-amp-replace (see example below). AMP will not replace variables that are not explicitly specified.

Here's an example of how inputs are before and after substitutions (note that you need to use platform syntax of variable substitutions and not analytics ones):

<!-- Initial Load -->
<form ...>
  <input
    name="canonicalUrl"
    type="hidden"
    value="The canonical URL is: CANONICAL_URL - RANDOM - CANONICAL_HOSTNAME"
    data-amp-replace="CANONICAL_URL RANDOM"
  />
  <input
    name="clientId"
    type="hidden"
    value="CLIENT_ID(myid)"
    data-amp-replace="CLIENT_ID"
  />
  ...
</form>

Once the user tries to submit the form, AMP will try to resolve the variables and update the fields' value attribute of all fields with the appropriate substitutions. For XHR submissions, all variables are likely to be substituted and resolved. However, in non-XHR GET submissions, values that requires async-resolution might not be available due to having not been resolved previously. CLIENT_ID for example would not resolve if it wasn't resolved and cached previously.

<!-- User submits the form, variables values are resolved into fields' value -->
<form ...>
  <input
    name="canonicalUrl"
    type="hidden"
    value="The canonical URL is: https://example.com/hello - 0.242513759125 - CANONICAL_HOSTNAME"
    data-amp-replace="CANONICAL_URL RANDOM"
  />
  <input
    name="clientId"
    type="hidden"
    value="amp:asqar893yfaiufhbas9g879ab9cha0cja0sga87scgas9ocnas0ch"
    data-amp-replace="CLIENT_ID"
  />
  ...
</form>

Note how CANONICAL_HOSTNAME above did not get replaced because it was not in the allowlist through data-amp-replace attribute on the first field.

Substitutions will happen on every subsequent submission. Read more about variable substitutions in AMP.

Autoexpand

AMP Form provides an autoexpand attribute to <textarea> elements. This allows the textarea to expand and shrink to accomodate the user's rows of input, up to the field's maximum size. If the user manually resizes the field, the autoexpand behavior will be removed.

<textarea autoexpand></textarea>

Polyfills

The amp-form extension provide polyfills for behaviors and functionality missing from some browsers or being implemented in the next version of CSS.

Invalid submit blocking and validation message bubble

Browsers that use webkit-based engines currently (as of August 2016) do not support invalid form submissions. These include Safari on all platforms, and all iOS browsers. The amp-form extension polyfills this behavior to block any invalid submissions and shows validation message bubbles on invalid inputs.

User-interaction pseudo-classes

The :user-invalid and :user-valid pseudo classes are part of the future CSS Selectors 4 spec and are introduced to allow better hooks for styling invalid/valid fields based on a few criteria.

One of the main differences between :invalid and :user-invalid is when are they applied to the element. The :user-invalid class is applied after a significant interaction from the user with the field (e.g., the user types in a field, or blurs from the field).

The amp-form extension provides classes to polyfill these pseudo-classes. The amp-form extension also propagates these to the ancestor form. However, fieldset elements are only ever set to have class 'user-valid' to be consistent with browser behaviour.

<textarea> validation

Regular expression matching is a common validation feature supported natively on most input elements, except for <textarea>. We polyfill this functionality and support the pattern attribute on <textarea> elements.

Security considerations

Protecting against XSRF

In addition to following the details in the AMP CORS spec, please pay extra attention to the section on "Processing state changing requests" to protect against XSRF attacks where an attacker can execute unauthorized commands using the current user session without the user knowledge.

In general, keep in mind the following points when accepting input from the user:

• Only use POST for state changing requests.
• Use non-XHR GET for navigational purposes only (e.g. search).
  • Non-XHR GET requests are will not receive accurate origin/headers and backends won't be able to protect against XSRF with the above mechanism.
  • In general, use XHR/non-XHR GET requests for navigational or information retrieval only.
• Non-XHR POST requests are not allowed in AMP documents. This is due to inconsistencies of setting Origin header on these requests across browsers, and the complications supporting it would introduce in protecting against XSRF. This might be reconsidered and introduced later — please file an issue if you think this is needed.

Attributes

target

Indicates where to display the form response after submitting the form. The value must be _blank or _top.

action

Specifies a server endpoint to handle the form input. The value must be an https URL (absolute or relative) and must not be a link to a CDN.

• For method=GET: use this attribute or action-xhr.
• For method=POST: use the action-xhr attribute.

action-xhr

Specifies a server endpoint to handle the form input and submit the form via XMLHttpRequest (XHR). An XHR request (sometimes called an AJAX request) is where the browser would make the request without a full load of the page or opening a new page. Browsers will send the request in the background using the Fetch API when available and fall back to XMLHttpRequest API for older browsers.

This attribute is required for method=POST, and is optional for method=GET.

The value for action-xhr can be the same or a different endpoint than action and has the same action requirements above.

To learn about redirecting the user after successfully submitting the form, see the Redirecting after a submission section below.

enctype

The enctype attribute specifies how form-data should be encoded before sending it to the server via the method=POST submission. The default encoding is set to multipart/form-data. This and application/x-www-form-urlencoded encoding types are currently supported.

Summary of enctype values:

• application/x-www-form-urlencoded - Sets the encoding type to application/x-www-form-urlencoded.
• multipart/form-data - Sets the encoding type to multipart/form-data.
• any value or unspecified - Setting the enctype attribute to a value not specified above or not setting the attribute at all will result in the default encoding type of multipart/form-data.

data-initialize-from-url

Initializes form fields from the window URL's search string, where the query parameter name matches the field's name. When this attribute is present, <input>, <select>, and <textarea> fields can optionally be initialized.

Individual fields are opted-in if they contain attribute data-allow-initialization. On page load, if the URL contains a query parameter with name matching an opted-in field's name attribute, the value or checked-state of that field will be updated based on the value of that query parameter. For example, <input type="search" name="q" data-allow-initialization> would display "my search" if the AMP page is visited with URL https://example.com/search?q=my+search.

Sample:

<form data-initialize-from-url
    method="get"
    action="https://example.com/search"    target="_top">
  <label>Search: <input type="search" name="q" data-allow-initialization></label>
</form>
<!-- display search results using amp-list -->

Limitations:

• Supported <input> types include checkbox, color, date, datetime-local, email, hidden, month, number, radio, range, search, tel, text, time, url, and week.
• Fields that take advantage of variable substitutions (fields with attribute data-amp-replace) are not supported.
• This feature is not supported on AMP pages delivered by an AMP cache. The data-initialize-from-url and data-allow-initialization attributes will not cause AMP validation failures, but the form fields will not be initialized from the URL.

xssi-prefix

Specifies that a prefix should be stripped prior to parsing the fetched json from the action-xhr endpoint. If the prefix is not present in the response, then this attribute will have no effect. This can be useful for APIs that include security prefixes like )]} to help prevent cross site scripting attacks.

Other form attributes

All other form attributes are optional.

custom-validation-reporting

This is an optional attribute that enables and selects a custom validation reporting strategy. Valid values are one of: show-first-on-submit, show-all-on-submit or as-you-go.

See the Custom Validation section for more details.

Actions

The amp-form element exposes the following actions.

submit

Allows you to trigger the form submission on a specific action, for example, tapping a link, or submitting a form on input change.

clear

Empties the values from each input in the form. This can allow users to quickly fill out forms a second time.

Events

Use amp-form events with the on attribute

The following example listens to both the submit-success and submit-error events and shows different lightboxes depending on the event:

<form
  ...
  on="submit-success:success-lightbox;submit-error:error-lightbox"
  ...
></form>

submit

The form is submitted and before the submission is complete.

submit-success

The form submission is done and the response is a success.

submit-error

The form submission is done and the response is an error.

verify

Asynchronous verification is initiated.

verify-error

Asynchronous verification is done and the response is an error.

valid

The form's validation state changes to "valid" (in accordance with its reporting strategy).

invalid

The form's validation state to "invalid" (in accordance with its reporting strategy).

Input events

AMP exposes change and input-debounced events on child <input> elements. This allows you to use the on attribute to execute an action on any element when an input value changes.

For example, a common use case is to submit a form on input change (selecting a radio button to answer a poll, choosing a language from a select input to translate a page, etc.).

<form id="myform"
    method="post"
    action-xhr="https://example.com/myform"    target="_blank">
    <fieldset>
      <label>
        <input name="answer1"
          value="Value 1"
          type="radio"
          on="change:myform.submit">Value 1
      </label>
      <label>
        <input name="answer1"
          value="Value 2"
          type="radio"
          on="change:myform.submit">Value 2
      </label>
    </fieldset>
  </form>

See the full example here.

Analytics

The amp-form extension triggers the following events that you can track in your amp-analytics config:

You can configure your analytics to send these events as in the following example:

<amp-analytics>
  <script type="application/json">
    {
      "requests": {
        "event": "https://www.example.com/analytics/event?eid=${eventId}",
        "searchEvent": "https://www.example.com/analytics/search?formId=${formId}&query=${formFields[query]}"
      },
      "triggers": {
        "formSubmit": {
          "on": "amp-form-submit",
          "request": "searchEvent"
        },
        "formSubmitSuccess": {
          "on": "amp-form-submit-success",
          "request": "event",
          "vars": {
            "eventId": "form-submit-success"
          }
        },
        "formSubmitError": {
          "on": "amp-form-submit-error",
          "request": "event",
          "vars": {
            "eventId": "form-submit-error"
          }
        }
      }
    }
  </script>
</amp-analytics>

All three events generate a set of variables that correspond to the specific form and the fields in the form. These variables can be used for analytics.

For example, the following form has one field:

<form action-xhr="/comment" method="POST" id="submit_form">
  <input type="text" name="comment" />
  <input type="submit" value="Comment" />
</form>

When the amp-form-submit, amp-form-submit-success, or amp-form-submit-error event fires, it generates the following variables containing the values that were specified in the form:

• formId
• formFields[comment]

Styling

Classes and CSS hooks

The amp-form extension provides classes and CSS hooks for publishers to style their forms and inputs.

The following classes can be used to indicate the state of the form submission:

• .amp-form-initial
• .amp-form-verify
• .amp-form-verify-error
• .amp-form-submitting
• .amp-form-submit-success
• .amp-form-submit-error

The following classes are a polyfill for the user interaction pseudo classes:

• .user-valid
• .user-invalid

Publishers can use these classes to style their inputs and fieldsets to be responsive to user actions (e.g., highlighting an invalid input with a red border after user blurs from it).

See the full example here on using these.

Validation

See amp-form rules in the AMP validator specification.