How can I properly specify the type of a function for a complex object with index signatures in TypeScript?

Problem with Retrieving Specific Data from Mixed Object

I'm encountering an issue with a function that is supposed to retrieve a specific piece of data within an object. The object contains a combination of known indexes and index signatures, which seems to be causing confusion for the function. I may have made a mistake in how I defined the function or perhaps there's a bug in the TypeScript linter. Any assistance on resolving this typing problem would be greatly appreciated.

The Function Definition

function getCounts<Entity extends EntityType>(entity: Entity, id: string): Counts[Entity][string] {
  return counts[entity][id] ?? []; // triggers an error
}

Error Message Received

Type 'CountsWithEntity<Entity1> | CountsWithEntity<Entity2>' is not assignable to type 'Counts[Entity][string]'.
  Type 'CountsWithEntity<Entity1>' is not assignable to type 'Counts[Entity][string]'.

Despite the Error, it Works!

Here's the proof

Correct Inferred Type during Execution

Check it out here

Playground Link to Reproduce the Issue

Access TypeScript Playground

Complete Code Snippet (same as above link)

[Code snippet provided]
typescripttypescript-typingstypescript-generics

Answer №1

The error message indicates that the return type required should be 

CountsWithEntity<Entity1> | CountsWithEntity<Entity2>
.

This is because the data type of counts[entity][id] is 

CountsWithEntity<EntityMap[entityKey]>

  • Here's a suggestion to address this issue
function getCounts<Entity extends EntityType>(entity: Entity, id: string): CountsWithEntity<Entity1> | CountsWithEntity<Entity2> {
  return counts[entity][id] ?? []; // triggering the error
}

IN ADDITION

You can also consider using the more dynamic return type 

CountsWithEntity<EntityMap[EntityType]>
.

function getCounts<Entity extends EntityType>(entity: Entity, id: string): CountsWithEntity<EntityMap[EntityType]> {
  return counts[entity][id] ?? []; // causing the error
}

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