JSONObject should support narrowing to `[key: number]:JSONValue`

[codyfrisch]

Use case

Behind the scenes, javascript converts numeric keys to strings. The pattern object[1234] is valid. However, JSONObject is defined as [key: string]: JSONValue and the result of getMultiple (dynamodb provider) cannot be cast into a type like [key: number] : JSONValue since string cannot be narrowed to number.

The type being used expects it to be a number, because that is how the parameter is accessed.

You can do object[1234] today without this change because TypeScript knows about the behavior of numbers being converted to strings. This is simply about narrowing the type to only allow numbers.

Solution/User Experience

Update JSONObject to the following type: type JSONObject = { [key: string | number]: JSONValue; }

This way a type can narrow JSONObject to [key: number]:JSONValue.

Alternative solutions

No response

Acknowledgment

• This feature request meets Powertools for AWS Lambda (TypeScript) Tenets
• Should this be considered in other Powertools for AWS Lambda languages? i.e. Python, Java, and .NET

Future readers

Please react with 👍 and your use case to help us understand customer demand.

[am29d]

Hey @codyfrisch

thanks for opening this issue!

Do I understand correct that you are using Parameters utility with DynamoDBProvider and want to fetch multiple parameters? Can you share a bit more details about your use case and the issue you have encountered? An example would help.

Thanks a lot.

[codyfrisch]

I haven't actually had an issue using the Parameters utility, from a functionality standpoint, its working fantastic.

This is entirely a types issue.

The JSONObject is as follows:

type JSONObject = { [key: string]: JSONValue; } today.

my parameters though are type MyParameters = { [key: number] : ParameterValue; }

When I do a getMultiple for the parameter, the object returned is of type JSONObject. I need to cast this into the type MyParameters. Since my type uses [key: number] (as my keys are always numbers) this causes a typescript error:

Conversion of type 'string | number | boolean | JSONObject | JSONArray' to type 'MyParameters' may be a mistake because neither type sufficiently overlaps with the other.

As soon as I change MyParameters to [key: string] this stops. Basically, my need is that I should be able to narrow the key of the parameter to a number (which JS converts to a string behind the scenes of course). This way type checking will help catch if I were to instead of a number try to get a parameter with a string like stringParameterId.

I'm not a typescript guru though so my supposition on how to correct it may be wrong. I am working around this with "this.myParameters = parameters as unknown as MyParameters` But my instincts tell me that some improvements to the JSONObject type could eliminate this need.

This is mostly just a suggestion to investigate an improvement to the typing.

[am29d]

Hey @codyfrisch ,

I have double checked the option to use number as key, and you are correct. We could extend the key with number, so you can catch it for your return value. I have tested this options in multiple e2e tests and it works.

Also feel free to submit a PR, if you'd like.