Lambda@Edge A/B testing - Origin Response

originResponse.js

'use strict';

const sourceCoookie = 'X-Source';
const sourceMain = 'main';
const sourceExperiment = 'experiment';
const cookiePath = '/';

// Origin Response handler
exports.handler = (event, context, callback) => {
    const request = event.Records[0].cf.request;
    const requestHeaders = request.headers;
    const response = event.Records[0].cf.response;    

    const sourceMainCookie = `${sourceCoookie}=${sourceMain}`;
    const sourceExperimenCookie = `${sourceCoookie}=${sourceExperiment}`;    

    // Look for Source cookie
    // A single cookie header entry may contains multiple cookies, so it looks for a partial match
    if (requestHeaders.cookie) {
        for (let i = 0; i < requestHeaders.cookie.length; i++) {    
            // ...ugly but simple enough for now   
            if (requestHeaders.cookie[i].value.indexOf(sourceExperimenCookie) >= 0) {
                console.log('Experiment Source cookie found');
                setCookie(response, sourceExperimenCookie);
                callback(null, response);
                return;
            }
            if (requestHeaders.cookie[i].value.indexOf(sourceMainCookie) >= 0) {
                console.log('Main Source cookie found');
                setCookie(response, sourceMainCookie);
                callback(null, response);
                return;
            }            
        }
    }
    
    // If request contains no Source cookie, do nothing and forward the response as-is
    console.log('No Source cookie found');
    callback(null, response);
}

// Add set-cookie header (including path)
const setCookie = function(response, cookie) {
    const cookieValue = `${cookie}; Path=${cookiePath}`;
    console.log(`Setting cookie ${cookieValue}`);
    response.headers['set-cookie'] = [{ key: "Set-Cookie", value: cookieValue }];    
}

Hi,

Thanks for your amazing code !
However, I have an issue with the line 46 when I test it with a cookie (X-Source=main or X-Source=experiment)

Here is the error detail :
Invoke Error {"errorType":"TypeError","errorMessage":"Cannot read property 'headers' of undefined","stack":["TypeError: Cannot read property 'headers' of undefined"," at setCookie

Do you know where it comes from ? Thanks a lot :)