CORS Preflight Autoresponder CloudFront Function

cors-preflight-autoresponder.js

/**
 * CloudFront function for responding to CORS Preflight requests at the edge.
 *
 * See: https://developer.mozilla.org/en-US/docs/Glossary/Preflight_request
 */
async function handler(event) {
    const request = event.request;
    if (request.method === 'OPTIONS' && request.headers.origin) {
        // see: https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/functions-event-structure.html#functions-event-structure-response
        const response = {
            statusCode: 200,
            statusDescription: 'OK',
            headers: {
                // header names must be lower-case in CF functions
                'access-control-allow-origin': {'value': request.headers.origin.value},
                'access-control-allow-methods': {'value': 'DELETE, GET, HEAD, OPTIONS, PATCH, POST, PUT'},
                'access-control-allow-headers': {'value': '*'},
                // https://developer.mozilla.org/en-US/docs/Web/HTTP/CORS#the_http_response_headers (Vary should be included)
                'vary': {'value': 'Origin'}
            }
        };
        return response;
    }
    return request;
}