Array.some should reflect that the array is not empty

[matthieusieben]

⚙ Compilation target

es5

⚙ Library

es5

Missing / Incorrect Definition

some in lib.es5.d.ts could be defined as follows (return type from boolean to this is [T, ...T[]]):

interface Array<T> {
    some(predicate: (value: T, index: number, array: T[]) => unknown, thisArg?: any): this is [T, ...T[]];
}

Sample Code

playground

function requiresOneItem<T>(array: [T, ...T[]]) {}
function isFoo<T>(item: T) { return true }

const array = [1, 2]

if (array.some(isFoo)) {
  requiresOneItem(array) // Argument of type 'number[]' is not assignable to parameter of type '[number, ...number[]]'.
}


interface Array<T> {
    // Uncomment to test:
    // some(predicate: (value: T, index: number, array: T[]) => unknown, thisArg?: any): this is [T, ...T[]];
}

Documentation Link

N.A. Calling .some() on an array can only ever return true if the array is not empty.

[RyanCavanaugh]

Type guards must always return true if the object matches the specified type.

function requiresOneItem<T>(array: [T, ...T[]]) {}

function isEven(item: number) {
  return item % 2 == 0;
}

const array: [number, ...number[]] = [1, 2]

if (array.some(isEven)) {
  requiresOneItem(array)
} else {
  const n = array;
  // n: never, which is wrong
}

interface Array<T> {
    // Uncomment to test:
    some(predicate: (value: T, index: number, array: T[]) => unknown, thisArg?: any): this is [T, ...T[]];
}