AWS Lambda script to set static asset (image, audio, etc.) "Cache-Control" metadata in S3

index.js

const aws = require('aws-sdk')

const s3 = new aws.S3({ apiVersion: '2006-03-01' })

const svar_key = 'cache_control_script_version'
const svar_val = 7

exports.handler = async (event, context) => {
  // console.log('Received event:', JSON.stringify(event, null, 2))

  // Get the object from the event and show its content type
  const bucket = event.Records[0].s3.bucket.name
  const key = decodeURIComponent(event.Records[0].s3.object.key.replace(/\+/g, ' '))

  console.log('target object: ' + bucket + '/' + key)
  console.log(`fetching...`)
  const data = await s3.headObject({Bucket: bucket, Key: key}).promise()

  console.log('current Cache-Control: ', data.CacheControl)
  const prev_svar = parseInt(data.Metadata[svar_key], 10)
  console.log(`check: prev_svar ${prev_svar} >= svar ${svar_val}`)

  // typeof data.CacheControl != 'undefined'
  if (!!prev_svar && prev_svar >= svar_val) {
    console.log('skipped: cache control already exists')
    return context.done(null, 'skipped')

  } else {
    let ttl = 0
    if (key.match(/\.(css|js)$/)) {
      ttl = 300

    } else if (key.match(/\.(ttf|woff|woff2|otf)$/)) {
      ttl = 86400

    } else if (key.match(/favicon\.(ico|png)$/)) {
      ttl = 86400

    } else if (data.ContentType.match(/^image\//)) {
      ttl = 300

    } else if (data.ContentType.match(/^audio\//)) {
      ttl = 86400
    }

    if (ttl === 0) {
      console.log('no matching criteria for target Cache-Control')
      return context.done(null, 'skipped')
    }

    const params = {
      Bucket: bucket,
      CopySource: bucket + '/' + key,
      Key: key,
      CacheControl: `max-age=${ttl}`,
      MetadataDirective: "REPLACE",
      Metadata: Object.assign({}, data.Metadata, {
        [svar_key]: svar_val.toString(),
      }),
      ContentType: data.ContentType,
    }

    try {
      console.log(`setting TTL: ${ttl}`)
      await s3.copyObject(params).promise()

    } catch (e) {
      return context.done('error', `failed to update: ${e}`)
    }

    return context.done(null, 'object updated successfully')
  }
}