I have a scenario where I want to achieve something similar to the following:
type Whoozit = string | number
type WhoozitMap = {[key: string]: (x: Whoozit) => boolean}
const whoozit: WhoozitMap = {
cheese: (x: string) => x === 'cheese',
digit: (x: number) => x < 10
}
However, the above code doesn't work as intended. The type of the functions in the map must be Whoozit, not just string or number. So, there are two ways to fix this issue:
const whoozit: WhoozitMap = {
cheese: (x: Whoozit) => { if (isString(x)) { return x === 'cheese' } else { return false } },
digit: (x: number) => { if (isNumber(x)) { return x < 10 } else { return false } },
}
// Although this is a solution, it adds unnecessary complexity as
// my code runtime can handle selecting the correct key from the map
or
type WhoozitMap = {[key: string]: (x: any) => boolean}
// This approach is cleaner, but it sacrifices all type safety within the map
Is there a way to express what I want in Typescript? Specifically, the following code snippet should compile:
const whoozit: WhoozitMap = {
cheese: (x: string) => x === 'cheese',
digit: (x: number) => x < 10
}
... whereas this example should not be valid?
const whoozit: WhoozitMap = {
cheese: (x: string) => x === 'cheese',
list: (x: number[]) => x.includes(22) // ❌ number[] does not extend Whoozit
}
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