Feature request: provide custom functions for JMESPath expressions

[saragerion]

Use case

When making their functions idempotent, users can apply JMESPath selectors that will be used to filter the event payload and/or perform validations.

In Powertools for Python, the Idempotency utility features some helper functions that allow to extend the JMESPath spec and apply transformations to the result of a selection:

The functions are defined here and used in the search here. Powertools for Python also exposes a number of common payload expressions which require the functions and are related to AWS Lambda events.

Solution/User Experience

N/A

Alternative solutions

No response

Acknowledgment

• This feature request meets Lambda Powertools Tenets
• Should this be considered in other Lambda Powertools languages? i.e. Python, Java

[dreamorosi]

The module used to implement JMESPath functions introduced in #1376 (jmespath) doesn't support passing options / using custom functions:
jmespath/jmespath.js#30
jmespath/jmespath.js#25

There is however another package called @metrichor/jmespath that appears to be a spec-compliant rewrite of the project made in TS and that supports custom functions. This version appears to be more similar to the Python implementation as it also allows to precompile expressions for later usage.

We should evaluate how important this feature is and consider potentially this alternative library.