How to handle CORS for AMP

Question

I have a page that uses amp-list to dynamically list products using a JSON file. I have my directory structure as follows: *root*/amp/product-name/ and then within the product-name directory I have an index.html with the AMP HTML and a products.json which contains the product data that I display on page. Everything is working as expected if I visit the page directly: https://example.com/amp/product-name/

The problem comes in when I visit from the ampproject.org CDN.

Here's the actual page, if you visit the console you can see there is a CORS error. How can I handle this? Documentation is not clear on how to handle exactly, just a very vague overview, so for someone new this doesn't make sense to me and I can't just intuit how to do it. I know I need to have a request handler of some sort, which I've partially made by copying the code from AMP's own app.js they link to in their documentation. Within the amp-list element I previously had the src pointed directly to the JSON file, but now I'm seeing that I need to point to a handler (like a JS file) and then have the handler set the request header and then output the proper JSON.

Here's what I have for the request handler:

/**
 * @param {string} url
 * @param {string} param
 * @param {*} value
 * @return {string}
 */
function addQueryParam(url, param, value) {
  const paramValue = encodeURIComponent(param) + '=' + encodeURIComponent(value);
  if (!url.includes('?')) {
    url += '?' + paramValue;
  } else {
    url += '&' + paramValue;
  }
  return url;
}

function enableCors(req, res, origin, opt_exposeHeaders) {
  res.setHeader('Access-Control-Allow-Credentials', 'true');
  res.setHeader('Access-Control-Allow-Origin', origin);
  res.setHeader('Access-Control-Allow-Origin', 'https://www-perfectimprints-com.cdn.ampproject.org');
  res.setHeader('Access-Control-Expose-Headers', ['AMP-Access-Control-Allow-Source-Origin'].concat(opt_exposeHeaders || []).join(', '));
  if (req.query.__amp_source_origin) {
    res.setHeader('AMP-Access-Control-Allow-Source-Origin', req.query.__amp_source_origin);
  }
}

function assertCors(req, res, opt_validMethods, opt_exposeHeaders, opt_ignoreMissingSourceOrigin) {
  // Allow disable CORS check (iframe fixtures have origin 'about:srcdoc').
  if (req.query.cors == '0') {
    return;
  }
  const validMethods = opt_validMethods || ['GET', 'POST', 'OPTIONS'];
  const invalidMethod = req.method + ' method is not allowed. Use POST.';
  const invalidOrigin = 'Origin header is invalid.';
  const invalidSourceOrigin = '__amp_source_origin parameter is invalid.';
  const unauthorized = 'Unauthorized Request';
  let origin;
  if (validMethods.indexOf(req.method) == -1) {
    res.statusCode = 405;
    res.end(JSON.stringify({
      message: invalidMethod
    }));
    throw invalidMethod;
  }
  if (req.headers.origin) {
    origin = req.headers.origin;
    if (!ORIGIN_REGEX.test(req.headers.origin)) {
      res.statusCode = 500;
      res.end(JSON.stringify({
        message: invalidOrigin
      }));
      throw invalidOrigin;
    }
    if (!opt_ignoreMissingSourceOrigin && !SOURCE_ORIGIN_REGEX.test(req.query.__amp_source_origin)) {
      res.statusCode = 500;
      res.end(JSON.stringify({
        message: invalidSourceOrigin
      }));
      throw invalidSourceOrigin;
    }
  } else if (req.headers['amp-same-origin'] == 'true') {
    origin = getUrlPrefix(req);
  } else {
    res.statusCode = 401;
    res.end(JSON.stringify({
      message: unauthorized
    }));
    throw unauthorized;
  }
  enableCors(req, res, origin, opt_exposeHeaders);
}

And here's what I'm using for the HTML.

<amp-list credentials="include" width="auto" height="1500" layout="fixed-height" src="https://perfectimprints.com/amp/handle-amp-requests.js" class="m1">
      <template type="amp-mustache" id="amp-template-id">
        <div class="product" style="padding-top: 1em;">
          <a class="wrapper-link" href="{{ link }}">
            <amp-img alt="{{ title }}" width="1000" height="1000" src="{{ src }}" layout="responsive"></amp-img>
            <h3 class="product-name text-centered">
              {{title}}
            </h3>
            <h4 class="text-centered sku margin-minus-1">{{ sku }}</h4>
            <div class="text-centered get-price">
              <a class="text-centered get-price-link" href="{{ link }}">Get pricing now</a>
            </div>
          </a>
        </div>
      </template>
      <div overflow role="button" aria-label="Show more">
        Show more
       </div>
    </amp-list>

https://www-perfectimprints-com.cdn.ampproject.org/c/s/www.perfectimprints.com/amp/trick-or-treat-bags/index.html

Answer 1

This was the solution that worked for me.

Header set Access-Control-Allow-Credentials "true"
Header set Access-Control-Allow-Origin "https://example-com.cdn.ampproject.org"
Header set Access-Control-Allow-Source-Origin "https://example.com"
Header set Access-Control-Expose-Headers AMP-Access-Control-Allow-Source-Origin
Header set AMP-Access-Control-Allow-Source-Origin "https://example.com"

Thank you to @Craig Scott for the prompt to help me solve.

Woe be the poor, inexperienced soul who deals with AMP pages.

Answer 2

Presuming you're on an Apache server, you could set your CORS header in your htaccess file like this:

Header set Access-Control-Allow-Origin "https://www-perfectimprints-com.cdn.ampproject.org"
Header set Access-Control-Allow-Origin "https://www-perfectimprints-com.amp.cloudflare.com"

Then ensure your htaccess file is uploaded to the directory that contains your website HTML. That's how it's set-up on the site I manage and everything works for us.

Answer 3

For people using nodejs, here's the syntax:

        res.set('AMP-Redirect-To', encodedUrl);
        res.set('Access-Control-Allow-Origin', websiteHostname);
        res.set('AMP-Access-Control-Allow-Source-Origin', websiteHostname);
        res.set('Access-Control-Expose-Headers', [
            'AMP-Access-Control-Allow-Source-Origin',
            'AMP-Redirect-To',
        ]);
        res.set('Access-Control-Allow-Credentials', 'true');

ِAnother solution is to use the amp-toolbox-cors library and set it's verifyOrigin option to false.

import ampCors from 'amp-toolbox-cors';

app.use(
    ampCors({
        verifyOrigin: false,
    }),
);

beware that all origins are allowed like that. In reality you should allow just two.

Hope it saves somebody's time in the future!