CloudFront Functions
Javascript Runtime
The CloudFront Functions Javascript runtime 2.0 is compliant with ECMAScript v5.1 and supports some features of v6 through to v12. There are some limitations which there seem to be no documentation for. You can't loop over more than 100 things (limit hit with loop over redirects).
See the AWS docs for the Javascript Runtime.
Monitoring
Metrics
Under CloudFront -> Telemetry -> Monitoring you can see metrics like throttles, errors and the cpu usage (scaled from 0 to 100). At or above 100% cpu usage, which is really execution time, your code will get terminated and the request will proceed to the CloudFront cache regardless. This means CF Functions should not be relied upon for any critical access control or restrictions on content.
Logs
CF Function logs are always sent to us-east-1. The log group name is in the format /aws/cloudfront/function/FunctionName, where FunctionName is the name that you gave to the function when you created it.
Quotas
Cloudfront Functions and KV Stores have more restrictive limits than Lambda@Edge, many of which cannot be increased.