handler.js

'use strict';

let aws = require('aws-sdk');
let s3 = new aws.S3({apiVersion: '2006-03-01'});

module.exports.cacheControl = (event) => {
    // Bucket name & File path
    const bucket = event.Records[0].s3.bucket.name;
    const key = decodeURIComponent(event.Records[0].s3.object.key).replace(/\+/g, ' ');

    let cacheDuration = 0;

    // Set image cache for 1 month
    if (key.match(/\.(?:jpg|jpeg|gif|png|ico|cur|gz|svg|svgz|mp4|ogg|ogv|webm|htc)$/)) {
        cacheDuration = 6696000;
    }

    // More checks for other file types (js, css, fonts, ..) here
    // ...

    // No cache needed, do nothing.
    if (cacheDuration <= 0) {
        return;
    }

    let cacheControlHeader = 'max-age=' + cacheDuration;

    // Get the file so we cacn check the meta data.
    s3.getObject(params, (err, data) => {
        // Something went wrong or file already has the cache control meta data, don't do anything.
        if (err || data.CacheControl == cacheControlHeader) {
            return;
        }

        let params = {
            Bucket: bucket,
            Key: key,
            CopySource: encodeURIComponent(bucket + '/' + key),
            ContentType: data.ContentType,
            CacheControl: cacheControlHeader,
            Metadata: {},
            MetadataDirective: 'REPLACE'
        };

        // Overwrite the file.
        s3.copyObject(params, (err, data) => {
            // Something went wrong, don't do anything.
            if (err) {
                return;
            }

            console.log('Metadata updated successfully!');
        });
    });
};