Unable to update the incoming CloudFront request using Lambda Edge

Question

I am working on a feature where the incoming requests from CloudFront are intercepted using Lambda Edge and then the program checks if there is a jpg file of same name present in S3. If yes, it updates the request to use jpg file otherwise use png file.

In the logs, I see a request coming in and the request getting updated with the jpg url in case a jpg file exists. But, on the webpage I still png file in every case.

Can anybody please advise what I am doing wrong?

Here's the code:

exports.handler = async (event, context, callback) => {

const request = event.Records[0].cf.request;
const headers = request.headers;
const origin = request.origin;

console.log("Original Request: " + request.uri);

if(request.uri.startsWith('/looks/') && request.uri.endsWith('.png')) {
    var jpg_file = request.uri.replace(".png", ".jpg");
    console.log("Modified Request: "+ jpg_file);
 
    var jpgFileExists = checkIfFileExists(s3, jpg_file.substr(1));

    jpgFileExists.then(function(result) {
        const type = result;
        
        console.log("TYPE: " + type);
        
        if(type) {
            console.log("send JPG file");
            request.uri = jpg_file;
        } else {
            console.log("send png file");
            //do nothing
        }
        
    }); 
}

console.log("FINAL URL: " + request.uri);

return callback(null, request);
};


async function checkIfFileExists(s3, fileName) {
let exist = true;
let params = {
    Bucket: "bucketName",
    Key: fileName
};

try {
    console.log("Checking for file: " + fileName);
    await s3.headObject(params).promise();
} catch (err) {
    exist = false;
}

console.log("EXISTS: " + exist);

return exist;

}