Add a cross-origin resource sharing (CORS) header to the response

The following viewer response function adds an Access-Control-Allow-Origin HTTP header to the response if the response doesn’t already contain this header. This header is part of cross-origin resource sharing (CORS). The header’s value (*) tells web browsers to allow code from any origin to access this resource. For more information, see Access-Control-Allow-Origin on the MDN Web Docs website.

See this example on GitHub.

JavaScript runtime 2.0

async function handler(event)  {
    const request = event.request;
    const response  = event.response;
 
    // If Access-Control-Allow-Origin CORS header is missing, add it.
    // Since JavaScript doesn't allow for hyphens in variable names, we use the dict["key"] notation.
    if (!response.headers['access-control-allow-origin'] && request.headers['origin']) {
        response.headers['access-control-allow-origin'] = {value: request.headers['origin'].value};
        console.log("Access-Control-Allow-Origin was missing, adding it now.");
    }

    return response;
}

JavaScript runtime 1.0

function handler(event)  {
    var response  = event.response;
    var headers  = response.headers;

    // If Access-Control-Allow-Origin CORS header is missing, add it.
    // Since JavaScript doesn't allow for hyphens in variable names, we use the dict["key"] notation.
    if (!headers['access-control-allow-origin']) {
        headers['access-control-allow-origin'] = {value: "*"};
        console.log("Access-Control-Allow-Origin was missing, adding it now.");
    }

    return response;
}