identity-function.ts

Constrained Identity Function


- Use Case: We want to create a Record with a limited set of Keys, and a defined type of Values.
	- We also don't want to Type the Set of Keys, but leverage TypeScript inference instead (DRY)
- If we create a new inline object literal, TypeScript will prevent any new key from being added, by won't enforce the type of the Values.
	- So Narrow Keys, Wide Value Type
- If we specify the type Record<string, number>, we'll be able to add any new Key after the object creation
	- So Wide Keys, Narrow Value Type
- By using a Types Identity Function, we can build an object with enforced Value Type, and Narrow set of Keys.

```
    const createObject = <ObjectType extends Records<string, number>> (item: ObjectType) => item
    const object = createObject({ x: 1, y: 2, z: "3" // Type Error })

    object.w = 5 // Error, key doesn't exist
```