Issue with TypeScript version 4.2.1 - The Array.some() method does not support a function that returns a boolean

I encountered a TypeScript error that goes as follows:

The complete error message reads:

Supplied parameters do not match any signature of call target: parameter type mismatch. > Parameter 'Predicate' should have type assignable to {(value: TrackedItem, index: number, array: TrackedItem[]): typeof unknown; }' but it has type {(x: TrackedItem): boolean; }'

Why do these seemingly compatible variants not align?

At this juncture, my inclination is towards speculating that the TypeScript typing being recognized is outdated or faulty. I observed that switching it to this configuration resolves the issue:

this.trackedItems().some((x => !x.isPutaway()) as any)
typescript

Answer №1

unknown type made its debut in Typescript 3.0. If your version of Typescript is below 3.0, this could be the issue causing the error.

The error message may vary slightly from what you expect. You might anticipate seeing: 

(value: TrackedItem, index: number, array: TrackedItem[]) => unknown
but instead receive 
(value: TrackedItem, index: number, array: TrackedItem[]): typeof unknown
. In your scenario, unknown is being used as a value when it should be used as a type.

This was tested on Typescript 4.3.2:

console.clear()

interface TrackedItem {
  item: number
}

const items: TrackedItem[] = [
  {
    item: 1
  },
  {
    item: 2
  }
]

const hasSome: boolean = items.some(x => x.item === 1)

console.log(hasSome) // prints true

// for:
//
// const hasSome: boolean = items.some((x: TrackedItem, y: string) => true)
//
// we get:
//
// Argument of type '(x: TrackedItem, y: string) => boolean' is not assignable to parameter of type '(value: TrackedItem, index: number, array: TrackedItem[]) => unknown'.
//   Types of parameters 'y' and 'index' are incompatible.
//     Type 'number' is not assignable to type 'string'.(2345)

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