Addressing Complexity
This code may seem simple, but the use case it serves is quite complex:
type CustomFunction = <B extends ['A']>(
param?: B
) => B extends undefined ? 'TYPE_1' : 'TYPE_2'
// Usage examples:
const example1 = customFn() // Returns TYPE_1
const example2 = customFn(['A']) // Returns TYPE_2
- I want the type to be 'TYPE_1' if
paramis undefined, and 'TYPE_2' otherwise. paramshould have dynamic typing, meaning that when values are written within the array, the IDE should provide autocomplete and validation support.- The type of
paraminside the function should be at least (string[] | undefined).
Exploring Different Approaches
I've attempted various configurations, but none of them were successful. You can experiment with these failed attempts in the TypeScript Playground.
The closest I got was with this code:
type MyFn = <
T,
B extends ['A']
>(
param?: T | B
) => T extends string[] ? 'TYPE_2' : 'TYPE_1'
var a: MyFn3 = (param?) => (param ? 'TYPE_1' : 'TYPE_2') as any
const test1 = a() // Returns TYPE_1
const test2 = a(['A']) // Returns TYPE_2
The Dilemma: The inferred type inside the function is any because T has no constraint. However, when we add constraints to the T type, the condition in the return type always evaluates to true.
Please Rescue Me!: Is there any solution that can help overcome this problem?
A Practical Application
This custom type serves the purpose of defining an array of strings as props for addressing an object and returning the corresponding value.
For example, given ['level1', 'level2'], the function should return myObject.level1.level2.